Dr. Rebecca Dekker – 00:01:40:

Before we dive into today’s very important episode, I wanted to share an exciting announcement. And that is, that tickets for the virtual 2026 Evidence Based Birth® conference are officially open today for the waitlist only. A few weeks ago I gave you a hint as to what one of our special evidence topics will be at the conference. I had said that we were covering a hot topic that begins with the letter “H.” Today, I can officially let you know that at the EBB conference, the EBB research team will be giving a brand new presentation covering the evidence on the Hepatitis B vaccine for newborns. This topic has been highly requested. So, if you want a front row seat at our virtual conference, go to ebbirth.com/waitlist to join the waitlist today for access to early bird tickets and a special post-conference bonus Q&A session with me. So, only those of you who are on the waitlist will receive an exclusive invitation to attend that bonus session. This is one of the perks the waitlist receives in addition to early bird pricing, when you register for the conference by Tuesday January 27th. If you have not had a chance to join the waitlist, but you’re really interested in this conference, and you would like early bird pricing and an invitation to the Q&A session with me – you can sign up at ebbirth.com/waitlist. After joining the waitlist the registration link will be sent immediately to your inbox. You can take advantage to the discounted pricing and the waitlist only bonus. And now, let’s get to today’s episode. 

Hi everyone. On today’s podcast, we’re going to talk about the evidence on “What is a due date?” Welcome to the Evidence Based Birth® Podcast. My name is Rebecca Dekker, and I’m a nurse with my PhD and the founder of Evidence Based Birth®. Join me each week as we work together to get evidence-based information into the hands of families and professionals around the world. As a reminder, this information is not medical advice. See ebbirth.com/disclaimer for more details. Hi everyone, and welcome to today’s episode of the Evidence Based Birth® Podcast. Today I have a co-host with me, Dr. Sara Ailshire from Team EBB, is here with me to talk about some brand new evidence-based information about due dates. Dr. Sara has been on the podcast many times before, and I’m so excited to welcome you back.

Dr. Sara Ailshire – 00:02:36:

Hi, thank you so much for having me back on. I’m really excited to talk about everything that we’ve added to the article and the latest research on this topic. Updating these Signature Articles is a really big task, and it’s really fun to get to this point where we can actually share everything that we’ve been working on.

Dr. Rebecca Dekker – 00:02:52:

Yeah, and before we dive into the research, I just want to let you all know that if you want to access any of the written materials or scientific references that we mentioned on the podcast, it’s really easy. Just go to ebbirth.com/duedates and everything is there waiting for you. So to give a little bit of background, I originally published the Evidence on Due Dates in 2015. And we last updated that information in 2019. So the last podcast episode on this was way back in episode 51 that was published in 2018. So we’re recording this and this is being published in 2026. So this is a really big update for you all about due dates. We actually have a lot of new content coming out this year about due dates and induction. So you’re going to be hearing today about the Evidence on Due Dates. Later on this spring, we will be updating the Evidence on Induction for Due Dates. And we’ll be releasing that companion article to this one later this year. So as a content note, in today’s episode, we will be discussing stillbirth, which is defined as the loss of a baby at or beyond 20 weeks of pregnancy. Stillbirth touches the lives of so many birth workers, parents, and providers. And in the show notes, we’re going to share some resources that honor those babies that were lost to stillbirth and support parents and families affected by that type of loss. So with due dates, who might be interested in listening to this episode? What is there to learn about the science of due dates? Because doesn’t every pregnancy have roughly the same estimated length of 40 weeks? And why are some healthcare providers so concerned about pregnancies going past term? So in today’s episode, we’re hopefully going to be helping out those of you who might be pregnant now, or maybe you’re a healthcare provider or a birth worker. We’re going to cover what are due dates and what we know about how to date a pregnancy. We’re going to talk a little bit about ultrasounds, how they work, which is an important way that many pregnancies are dated. We will also talk about the risks of stillbirth at the end of pregnancy, what the latest research says on this, and what experts say you can do to help lower the risk of stillbirth. And of course, we’ll end by talking about the bottom line. So Dr. Sara, can you start us off by explaining what is a due date?

Dr. Sara Ailshire – 00:05:24:

Yeah, working on this update was the first time I’d ever really had a chance to sit down and dig into what we know about how long a pregnancy should last. And I think the first thing I should say is that a due date is really an estimate. It’s an educated guess, but it’s a guess for about how long a person could expect to be pregnant. It’s less important as a date of when you should expect to have a baby. It’s more helpful for helping you identify a range of dates. And when you’ll be at term, which is between 37 weeks and zero days pregnancy, an early term up to 41 weeks, six days for late term, and sometimes beyond. So generally speaking, the estimated due date is traditionally 40 weeks and zero days after the first day of your last menstrual period.

Dr. Rebecca Dekker – 00:06:16:

Okay. And so how does someone calculate that?

Dr. Sara Ailshire – 00:06:19:

So almost everybody, so doctors, midwives, and even those like online due date calculators you might use, they all use something called Naegele’s Rule to figure out the estimated due date. Naegele’s Rule assumes that you have a 28 day menstrual cycle and that you ovulate exactly on the 14th day of your cycle. And I should note that some healthcare providers will be able to adjust your due date for a longer or shorter menstrual cycle. So for a little bit of math to calculate your estimated due date or your EDD, using Naegele’s rule, you’ll add seven days to the first day of your last period and then count for nine months or you could count backwards three months. So that’s about equivalent to 280 days from the start of your last period. So say you have your last menstrual period on April 4th, you’ll add seven days. That brings you up to April 11th. Then you can subtract three months or go forward nine months. Either way, that’ll bring you to an estimated due date of January 11th. Another way to think about it is saying that your estimated due date is 40 weeks after the first day of your last period. So in cases where you know exactly when you conceive, so like with IVF or if you’re a very diligent fertility tracker where you might know your ovulation day, you can calculate the estimated due date by adding 266 days to the known date of conception or by subtracting seven days and adding nine months. This can increase the accuracy of that guess because it no longer assumes a 14-day ovulation based on the first day of the last menstrual period. But if you’re listening to this and you’re like, okay, Sara, but my menstrual cycle is like 30 days. That’s a really good point. And that is one of the big problems with Naegele’s rule. It assumes a lot of things that are not true for most people. Lots of people’s cycles vary in length. You know, when I was tracking mine prior to conception, I think I was averaging like 26 days, for example. So people go longer, well into 30 days, right? People can ovulate earlier or later. And sometimes even after the egg has been fertilized, it doesn’t automatically implant. Those eggs can wander around and take their time in there a little bit, right? So there’s a number of things that can affect, you know, the order of operations necessary for a pregnancy to begin. So that’s kind of where the, you know, tried and true numbers in Naegele’s Rule kind of fall apart, right? It’s a guess, but it’s not the best guess. So for this reason, an ultrasound between 11 and 14 weeks of gestational age is the best method that we currently have for accurately or most accurately, I should say, dating a pregnancy. And during this ultrasound, rather than sort of just using those dates of like when you had your period and the ultrasound technician will take measurements of the fetus. And that is, they’ll use those measurements to kind of say, okay, this baby, this fetus is about, you know, this many weeks along. And it can give you the best estimate that we currently have for how long the fetus has been developing or how long you’ve been pregnant. So generally speaking, a lot of people get that initial ultrasound to confirm the pregnancy, and then they’ll go back in. And that second ultrasound that you get, that’s your dating scan. And again, it’s not perfect, right? Because bodies are different. Babies are different. They’re very educated guesses, but just sort of like our best chance of kind of figuring out how long you could expect to be pregnant.

Dr. Rebecca Dekker – 00:09:45:

Yes, I think it’s really interesting how that ultrasound can be more accurate than using the last menstrual period to date your pregnancy. But it makes sense because there’s a lot of flaws, as you mentioned, with using the last menstrual period. But I think one of the most important studies on this was published back in 2013 by Khambalia et al. And it was a really high quality study, and they were grouping ultrasound scans by less than seven weeks, seven to 10 weeks, 11 to 14 weeks, and beyond. And they found that the most accurate time to perform an ultrasound to determine the gestational age was 11 to 14 weeks. So about 68% of people in the study gave birth within plus or minus 11 days of their estimated due date, as calculated by that ultrasound at 11 to 14 weeks. And this was more accurate than any of the other timings of ultrasounds and more accurate than using the last menstrual period. But the accuracy of ultrasound starts to significantly decline starting at about 20 weeks. And if you’re using a due date that was calculated beyond 20 weeks, you’re going to have a much higher risk of your pregnancy or when you go into labor as being inaccurately identified as either preterm or postterm. So that’s why most guidelines recommend that you use an 11 to 14 week dating ultrasound as the best practice for dating a pregnancy. Sara one of the things we did when updating this article, we’ve always talked about the accuracy of ultrasounds to date a pregnancy, but we have been getting a lot of questions from parents asking, is it safe to get an ultrasound to date the pregnancies? Can you talk a little bit about that?

Dr. Sara Ailshire – 00:11:22:

Yeah, absolutely. So I think we’re going to talk a little bit actually about what an ultrasound is. So ultrasound uses sound waves to create an image of what’s happening inside the body. And the image that you get is created by the ultrasound as a sonogram or like literally like a sound picture. That’s that little, you know, if you go, you get your ultrasound, you have your little bean or your little baby, like that’s the picture you have, right? Obstetric ultrasounds can tell us lots of information. They can help us, you know, make sure that organs are where they’re supposed to be. They can give information about, you know, how big the baby is. Not perfect, not a hundred percent. Of course, they can show us information about the placenta and other important body structures in pregnancy. And there’s lots of reasons why you would want to have an ultrasound during pregnancy. Use it to estimate gestational age. It can help us identify potentially life-threatening problems like placenta previa or placenta accreta, where there’s problems with the placenta’s connection or position in the uterus. It can help us detect differences of development in the fetus that can allow families to make informed medical decisions. It can provide a lot of reassurance to parents who’ve experienced pregnancy loss. It can facilitate bonding and it can, you know, again, give us a way to try to estimate or attempt to estimate fetal size. But some people prefer not to have ultrasounds during pregnancy. And that’s something that I learned as we were working on this update about why people are hesitant or concerned, like what the fear is around ultrasounds. So some people think that they’re unnecessary if they’re unwilling to terminate a pregnancy for any reason. Some people believe that ultrasounds have no value as a tool for monitoring a pregnancy’s progression or for diagnosing problems in a pregnancy. And some people believe that ultrasounds are harmful because of the sound waves that are used to generate the picture, the sonogram. So if you’re a person who’s in that boat or you’re, I’m not sure, I don’t know, like what happens with those sound waves. So you should be reassured to know that the research hasn’t identified any health risks of using standard ultrasound waves during pregnancy care. However, it’s not possible for us to prove the complete absence of risk because sound waves used in ultrasound scans are a form of energy that can create heat and can have a slight physical effect on tissues. It can be helpful to think about how extremely loud sounds can cause speakers to rattle or how a softer sound can make you feel you have a gentle vibration in your body. Because of the potential of sound to affect tissues, ultrasound professionals try to limit exposure to sound waves as much as they possibly can during an ultrasound by using a standard known as ALARA, or as low as is reasonably achievable. So ALARA, the standard includes really strict guidelines around sound wave like strength and the time limits during the ultrasound. So all this means is that ultrasound technicians use the lowest intensity of sound waves for the shortest amount of time possible to get a good picture. And this ALARA principle is actually also why professional bodies that regulate, you know, ultrasound and ultrasound technician training recommend against those keepsake ultrasounds. Many people, I should say, want to see that their baby looks like, but generally speaking, if there’s not a need, you know, either to like determine where you are in pregnancy or check, you know, make sure that everything is developing well, and that organs and placentas and everything are where they’re supposed to be, then it’s not recommended to have an ultrasound just sort of recreationally. So that was something I learned during this research, I thought was super interesting, because I was pregnant not that long ago. And like, we considered going in just, you know, you want to see how everyone’s doing in there. But if there’s not, you know, again, like a documented need for ultrasound, following that principle of, you know, reducing risk, even though we know that the risks are very, very low, that’s not something that’s recommended.

Dr. Rebecca Dekker – 00:15:18:

Yeah. So avoiding those like storefronts where you go in to just get like, feels like tourism of, you know, a picture with your baby is what should be avoided. But brief ultrasound to date the pregnancy is generally considered to be safe. And that’s widely accepted among healthcare organizations. So very interesting. And I do know that the dating ultrasounds tend to be a very short, quick, take a picture, measure the fetus. And I think it’s important to know that those measurements to date the pregnancy are going to be more accurate earlier in pregnancy because early in pregnancy, embryos and fetuses develop at a very specific timeline. But later in pregnancies, when you start to see differences in how we are built, you know, where maybe your partner is really tall. So your baby’s growing extra big. That doesn’t mean they’re older inside the uterus. It just means that’s what they’re going to end up being a taller person. So that’s one reason why the earlier, that 11 to 14 week period is the best time period to get the dating.

Dr. Sara Ailshire – 00:16:17:

Yeah, absolutely. So Rebecca, we talked a little bit about how we estimate pregnancy length and the markers that we use to plot our course. But 40 weeks, as we all know, isn’t a magic number. So would you be able to talk a little bit about the reality of how long pregnancies actually are compared to these estimates?

Dr. Rebecca Dekker – 00:16:38:

Yeah. So it’s actually kind of tricky to study the length of pregnancy. So a long time ago, researchers used to try and estimate how long the average pregnancy would go by, you take a large group of people. You’d measure the time from ovulation or from their last menstrual period or the date on an ultrasound. And then you would measure how long it would take for them to go into labor and give birth. And then you would just kind of calculate the average. But this method is completely inaccurate. And one of the reasons it’s inaccurate is because many people are induced maybe at 39 weeks or earlier, maybe 40, 41 weeks or later. And so if you’re including people who were induced in your average length of pregnancy, then you are including people who gave birth earlier than they would have if they hadn’t been induced because with an induction, they were not able to go into labor on their own timeline. And we have many, many people being induced today at 39, 40 weeks, especially after the ARRIVE trial, which we’ve talked a lot about on this podcast. But this really puts researchers in a bind because you can’t just throw out all those people who have inductions. For example, let’s say you exclude someone who was induced at 41 weeks from your study, then you’re ignoring a pregnancy that was induced because it was going longer. And so by excluding that case, you artificially make the average length of pregnancy too short. So how do we deal with this issue? There is a statistical method called time to event analysis, and sometimes it’s called survival analysis. This is a special method I used to use when I did cardiovascular research when we were looking at time to an event. How long does it take for something to occur? And it’s a special method that allows you to include everyone in your study and still get an accurate picture of how long it would take the average person to go into labor on their own. So there are two studies that have used this method. They’re both older studies. One was published by Smith in 2001. One was published by Jukic in 2013, but they’re still really relevant. You can throw that rule out, the rule that says only use a study if it was published in the last five years. Sometimes we can’t do that in birth work because there is no research or because we have a really like what we call a landmark study. And I don’t think the results will have changed, but what we have with these data, they’re really important. One study that was published by Smith, they had about 1500 healthy pregnant women. Their estimated due dates that were calculated by their last menstrual period were a perfect match with their first trimester ultrasound estimate. And they found that half of everyone in the study gave birth for the first time by 40 weeks and five days. So instead of using the average or the mean, we’re using the median, which is kind of like the halfway point. Well, 75% gave birth by 41 weeks and two days. Meanwhile, half of all women who had already given birth in the past at least once gave birth by 40 weeks and three days, while 75% gave birth by 41 weeks and zero days. So this means that 40 weeks and zero days was not like where most people went into labor. Actual pregnancy length was about five days longer than the traditional due date of 40 weeks, if you’re giving birth for the first time. And it was three days past 40 weeks if you had given birth before. So that was the Smith study. There was another study, it was a lot smaller, the Jukic study only had 125 healthy pregnant women in the study and they were giving birth like in the early 1980s. However, this is one of the most important studies we have on this topic because they began following these people even before they conceived. They were measuring their hormones daily for six months. So they knew like the exact day people ovulated, the exact day that conception happened and they even knew the exact day that the pregnancy implanted in the uterus. So after they excluded anybody with medical complications or preterm births, the final sample of 113 women had a median time from ovulation to birth of 38 weeks, two days after ovulation. But the median time from the first day of the last menstrual period to birth was 40 weeks, five days after the last menstrual period. And they also found that only 10% gave birth by around 39 weeks and half, like I said earlier, gave birth by 40 weeks, five days. 75% gave birth by 41 weeks and two days. And then they also looked at more specifically ovulation. We have that information in the Signature Article. So if you want to look at the statistics of like, when exactly did people tend to give birth after they ovulated and conceived, you can see that in the article as well. I thought it was really interesting that the longer the embryo took, like that fertilized egg, the longer it took to implant in the uterus to kind of embed itself in the uterus, the longer the pregnancy. So it’s not just the day you conceive, but it’s also the day the pregnancy implants. And so really based on best evidence, there is no such thing as an exact due date. It is just like you said, Sara, earlier, it is an educated guess. And it’s also more appropriate to say that there is a normal range of time in which most people will give birth. So about half of people will go into labor on their own by 40 weeks and five days if it’s their first baby, and about half will go into labor by 40 weeks, three days if you’ve given birth before. The other half would not, unless of course they were induced or had a Cesarean.

Dr. Sara Ailshire – 00:22:27:

Yeah, absolutely. And something that I thought was really interesting when we were doing this update is that there are some people who are just like compared to the average, more likely to have longer pregnancies. The most important predictor of this likelihood of having a longer pregnancy was having a family history of longer pregnancies. So that could be your own people who have mothers or sisters who have longer pregnancies, as well as the babies, other genetic parents, their family history. There’s a 2013 study by Oberg that looked at over 475,000 births in Sweden that were dated by ultrasound before 20 weeks. They found that genetics had an incredibly strong influence on your chance of having a birth after 42 weeks. Another thing that they looked at in the study was looking at trends and pregnancies in the same person. So in their study, they found that if you’ve had a post-term or birth after 42 weeks before, you’re 4.4 times more likely to have another post-term birth if you are pregnant with the same other genetic parent. If you’ve had a post-term birth before and you switch partners, you’re 3.4 times more likely of having a post-term birth with your new partner. And if your sister had a post-term birth then you’re 1.8 times more likely to have, again, like a post-term 42 week plus birth. So that’s a really strong influence because those very, very long pregnancies are not particularly common. Overall, researchers found that half of your chance of having a post-term birth comes from genetics. This includes the baby’s genetics, their genetic tendency to gestate for longer due to both the genes that they inherit from their birthing parent, as well as the other genetic parent, as well as the pregnant person’s genetic tendency to carry a pregnancy for longer. In addition to this study by Oberg, we found a number of other studies that covered factors that could cause a pregnancy to go for longer. And these included having a higher body mass index or BMI before you get pregnant, having just in general an increased weight gain during pregnancy. A longer time between when you ovulate and when your pregnancy implants. Older maternal age, so 35 plus. How heavy you were yourself as a baby, so heavier birth weight of the pregnant person. Having a higher education level. Being pregnant for the first time. Being pregnant with a male baby. Exposure to a stressful event. And these were studies that looked at things that included natural disasters and 9-11 in the United States later in pregnancy. Have some kinds of fetal anomaly or fetal genetic condition can cause a longer pregnancy. However, in a lot of cases, for all these different factors that were covered or identified as something that could increase the likelihood that you’d have a longer pregnancy. There’s a lot of cases where we just don’t know exactly why somebody might have a longer pregnancy or might have longer pregnancies compared to other people. So we know that some people have longer pregnancies than others. And we know that 40 weeks is just an educated guess. So what is a concern then that a lot of people hear about being pregnant beyond 40 or 41 weeks?

Dr. Rebecca Dekker – 00:25:41:

Yeah. So the primary concern with going beyond your due date is that the risk of complications increases beyond the 40 to 41 week mark. And in the full link Signature Article, you can get at ebbirth.com/due dates, we’re going to outline for you a few studies that going back the past 20 years that look at risks across different gestational ages of pregnancy, mainly looking at 37 weeks to 42 weeks and beyond. So some risks researchers identified as increasing as pregnancy continues from 37 to 42 weeks include infections such as chorioamnionitis or endomyometritis and placental abruption. A placental abruption is rare, but the risk does gradually go up. Different studies show a general increase in the likelihood of having a Cesarean or a vacuum or forceps assisted birth as pregnancy advances. The risk of hemorrhage also generally increases bit by bit. And some risks increase to a point and then seem to level off like the risk of preeclampsia. For babies, researchers have identified an increased chance of having moderate to thick meconium or stool in the amniotic fluid, which is the baby’s first poop happening before the baby is born. NICU admissions, the chance of having a big baby and the chance of your baby having low APGAR scores. But by far, the biggest concern researchers focus on with going past your due date is the risk of stillbirth. So I wanted to talk more in depth about that. And if you want some of the exact statistics of the other risks I’ve talked about, like I said, many of those are rare, but the risks gradually go up. You can see those in the full-length Signature Article. So with stillbirth, it’s really interesting because up until the 1980s, most researchers and many doctors thought that the risk of stillbirth at 41 to 42 weeks was really similar to the risk of stillbirth earlier in pregnancy. So there was not a lot of pressure to induce before the 1980s because they thought, you know, the risk does not go up. However, in 1987, a researcher published a paper basically explaining that the earlier researchers were all using the wrong math when they were calculating stillbirths. You know, whenever you’re trying to calculate a rate or a percentage, you have a numerator and you have a denominator. Basic fraction math, which, you know, if you haven’t been in elementary or middle school for a while, you have to go back and review. But when you’re looking at math, Dr. Yudkin found that they were all using the wrong denominator. When calculating the fraction, we don’t need to know how many stillbirths happen out of every 1000 births at 41 weeks. Instead, we need to know how many stillbirths happen at 41 weeks compared to everyone who’s pregnant or gives birth at 41 weeks. So you have to include all the healthy babies that are still living in utero in your denominator. So when you include everybody in the denominator that you’re supposed to, they found something very surprising. They found that the risk of stillbirth goes down throughout pregnancy and it reaches a low point at 37 to 38 weeks after which the risk starts to rise again. And this finding has been repeatedly found by different researchers in different countries all over the world at different time points. And it’s known as the U-shaped curve of stillbirth. So we have higher rates of stillbirth earlier in pregnancy, like closer you are to 20 weeks. Then they decline until you get to 37 to 38 weeks after which they rise again. Now the risk of stillbirth is still very low at 37, 38, 39, 40 weeks, even though it’s going up. But what happens is at 41 weeks, there seems to be a more substantial increase in the risk of stillbirth. And some researchers argue that although the physiological length of a pregnancy is around 40 weeks in three to five days, they say, well, 40 weeks and zero days may be the more functional length of a pregnancy. In their opinion, because of the increased risk of losing your baby, 40 weeks may be as long as the pregnancy should go. This is one of the major motivating factors behind the ARRIVE trial, looking at induction at 39 weeks, which has since led to an increase in inductions at 39 weeks without seeing any benefit to lowering stillbirth rates, including in the study itself. Now, in the article, we go into depth in some of the more recent studies about stillbirth rates, and you will find that these rates vary from study to study, depending on who they’re including in the study, and how they’re reporting the stillbirth rates, because there’s even more math they can do to look at the risk of stillbirth. In the United States, the stillbirth rate is 5.7 per 1,000 deliveries, or one out of every 175 pregnancies, according to the latest CDC data published in 2025. And although that risk may seem low overall, if you happen to be the family that experiences that one in 175 event, then the risk doesn’t seem so low to you personally anymore. So, Sara, I know you dug really deep into the research on stillbirth. Can you share some of what you found with us?

Dr. Sara Ailshire – 00:31:07:

Yeah, absolutely. So in the article, we included some summaries of large studies that looked at the risk of stillbirth after the estimated due date. And these were studies from Norway, Germany, and the U.S.. And again, kind of like we’ve discussed before, some of this data is older but still relevant. However, I wanted to talk about the most recent study that we looked at that was published in 2023. In this Signature Article, we included summaries of several large studies that looked at the risk of stillbirth after the due date. These studies come from Norway, Germany, and the U.S.. Of these studies, though, a 2019 study by Mugloo et al., remains one of the largest meta-analyses that we have on this topic. These researchers looked at 13 studies that included over 15 million pregnancies. And of the people, of the pregnancies included in their study, that also included 18,000 stillbirths. All of the studies that they looked at were carried out in four countries defined as being high income by the World Bank. So Denmark, Norway, U.K., and the U.S. So in this study, researchers stratified or organized the risk of stillbirth based on gestational week. So at 37 weeks, their research identified 0.11 stillbirths per 1,000 pregnancies and births. At 38 weeks, that increased to 0.16 per 1,000. At 39 weeks, 0.42 per 1,000. 40 weeks, 0.69 per 1,000. And here we see that jump that we talked about with a U-shaped curve. At 41 weeks, they saw 1.66 stillbirths per 1,000. And at 42 weeks, they saw 3.18 per 1,000. So based on their data, Mugloo et al., the researchers calculated the number needed to harm with waiting for labor to begin on its own. This number tells you how many people would need to wait for labor for one more week in order to experience one additional stillbirth. To experience one additional stillbirth, there would need to be at least 2,367 people waiting for labor for one more week beginning at 39 weeks. At 40 weeks, 1,449 people would have to wait for labor for one more week to experience one more additional stillbirth. At 41 and 42 weeks, those numbers drop. So at 41 weeks, only 604 people would need to wait one additional week to experience a stillbirth. And at 42 weeks, 315 people would have to wait for labor for one more week to experience an additional stillbirth. These researchers also found evidence that the healthcare system was failing Black mothers and Black babies. In the research study, Black mothers were 1.5 to 2 times more likely than White mothers to have a stillbirth at every week of pregnancy. When these researchers also looked at only low-risk pregnancies, they found a little bit of a difference in the risk of stillbirth. At 38 weeks, they saw 0.12 per 1,000. At 39 weeks, 0.14 per 1,000. At 40 weeks, 0.33 per 1,000. 41 weeks was 0.80 per 1,000. At 42 weeks. 0.88 per 1,000. Low-risk pregnancy in this study was defined as pregnancies that had a single baby with no congenital differences of development, no complications of pregnancy. They found no additional risk of newborn death when giving birth between 38 and 41 weeks, but the risk of newborn death did increase beyond 41 weeks. So although most researchers have found an increase in stillbirth rates in the late term and post-term period, some might consider the absolute increase in risk to be small until 41 weeks, after which it reaches between 0.80 and 1.66 out of 1,000 pregnancies and births, depending on the presence of other risk factors. So we know that generally speaking, stillbirth rates start to increase after 41 weeks, but there are also some other factors that can affect the risk of stillbirth.

Dr. Rebecca Dekker – 00:35:12:

And what are those?

Dr. Sara Ailshire – 00:35:13:

Yeah, so we identified a few factors in our update for the Signature Article. The first group of factors we can talk about are fetal risk factors. And these can include things like congenital abnormalities, male sex, RH incompatibility between the fetus and the pregnant person, and being small for gestational age. A 2023 research study reviewed data from over 125 million births across 13 countries. And this study identified being small for gestational age either at preterm, so before 37 weeks, or at term as being an important risk factor for stillbirth. However, I think it’s really important to note that being small isn’t necessarily a problem in and of itself. Rather, being small for gestational age can be an indication that there’s a health problem in a pregnancy or that something is going on with a placenta. So, sure, people having small babies is not necessarily surprising. But if the baby is much, much smaller than you would expect it to be, then that can be a sign that, hey, something might be going on. And this brings me to the second risk category that we looked at, which was placental risk factors. The placenta is the organ that nourishes the baby throughout pregnancy. So if problems arise with the placenta, the baby’s life support, or the umbilical cord that connects the baby to the placenta, then stillbirths can occur. So these can be emergent or emergency events, like the separation of the placenta from the uterus and the abruption. Or it can be chronic issues with the placenta, like premature aging. Conditions like preeclampsia, hypertension, or diabetes can also negatively impact the placenta’s ability to function, which can increase the risk of stillbirth. Another risk category that we identified were maternal risk factors. These are sometimes referred to as maternal characteristics that are associated with an increased risk of stillbirth. These can include things that no one can help, right? Like being pregnant with your first baby, having fewer than four prenatal visits, having no prenatal care, experiencing poverty, and having a high BMI. These can also include things like smoking, type 1 or type 2 diabetes, having hypertension prior to the beginning of pregnancy, being very young, so under the age of 18, or being over the age of 40, as well as having a history of previous stillbirth or being pregnant with more than one baby, like having twins or triplets. The final risk category that we wanted to highlight is racism. White supremacy is harmful to the well-being of pregnant people and to their babies. The most recent numbers that we have from the U.S., find that the overall stillbirth rate, so from 20 weeks onward, is more than twice as high for Black pregnant people than for white pregnant people, at 9.95 per 1,000 for Black pregnant people compared to 4.55 per 1,000 stillbirths for white pregnant people. Native Hawaiian and Pacific Islanders, American Indian and Alaska Native pregnant people also face disproportionately high rates of stillbirth based, again, on the latest data that we have in the U.S. So I know it can be hard to hear all of this. I found it to be really difficult myself when I was drafting the article update and preparing for the podcast episode. And something that we get asked when we’re talking about the topic of stillbirth is if there’s something that people can do. People want to do what they can if it’s possible to prevent this from happening. So Rebecca, could you talk a little bit about what pregnant people can do?

Dr. Rebecca Dekker – 00:38:37:

Yeah. So obviously there are stillbirths that are completely unpreventable and every stillbirth is a tragedy. And there are many factors that we have no control over when we’re pregnant. There are some things we can do to lower the risk that does not guarantee that you will give birth to a healthy baby, but there are some things that we can do to improve the chances that things will turn out okay. Some of the work to lower the risk for everyone has to involve addressing structural inequalities that worsen the risk of stillbirth for pregnant people of color, as well as for people who lack access to adequate prenatal care. So, you know, maternity care deserts, the closure of hospitals that make it difficult for people to access prenatal care. As we mentioned earlier, like having supportive, adequate prenatal care lowers the risk of stillbirth. Some other steps individuals can take to potentially lower the risk of stillbirth include not smoking and not consuming illicit drugs during pregnancy, accessing prenatal care regularly, as we talked about, including getting screened for conditions such as gestational diabetes and preeclampsia. So things like home blood pressure monitoring could be helpful in screening for preeclampsia. Seeking midwifery care, there’s research showing that midwifery care can lower the risk of stillbirth. And then we often get asked about sleeping and sleep positions during pregnancy. And there is evidence that sleeping on one side rather than on their back, as you get towards the second half of pregnancy is helpful for lowering the overall risk of stillbirth. We have talked before in the podcast about fetal movement tracking or kit counting, and there is still mixed evidence on the effectiveness of this. So the goal of tracking your baby’s movements would to help you be aware of your baby’s movements so that if you notice a decrease or a change in the typical movement pattern that you could seek additional help and testing to determine whether or not additional interventions are needed or just make sure you’re not developing a pregnancy complication. So if you’re interested in learning more about the evidence on kit counting, we do include a section in the Signature Article on that where you can dive deeper into that research. So while the evidence on tracking fetal movements is unclear, what is important is that if you feel something’s wrong or you notice a difference in fetal activity, it is a good idea to contact your provider or go to the labor and delivery ward for testing. And we will link in the show notes to some episodes we’ve done about this in the past. You can learn more about Count the Kicks organization and about the Push documentary about stillbirth. So those are just a few things. And again, I want to emphasize that you could do all of those things and still experience a tragedy. We don’t want to put any blame on people, but I also do want people to feel like, because knowledge is power, right? And sometimes people ask, is it helpful to do this? Is it helpful to do that? Yes, it might be helpful to sleep on your side, but don’t be ashamed if you wake up and you happen to be on your back. That’s not something that you can help. So I do remember there was an interesting study I saw years ago where they actually had people put a tennis ball in like a pantyhose and tie it around their waist so that the tennis ball was on their back. And they found that it helped people stay on their side, which makes sense. I have not seen further research on that, but I thought that was interesting at the time. So anyways, we get that question a lot. It can be helpful, but at the same time, we can’t control everything we do while we’re sleeping. So keep that in mind. And Sara, is there anything you want to talk about that you found surprising in this article? Because you spent months digging into this research on due dates, kind of looking at it with fresh eyes. What was your takeaway or your surprising finding?

Dr. Sara Ailshire – 00:42:37:

The only thing that surprised me the most was, I think, you know, being in this field for a while, like, you know, both from my dissertation research and then working at EBB, you know, I was well acquainted with, you know, 40 weeks isn’t set in stone. It’s an estimate. But understanding a little bit more of the history of, you know, how many efforts there have been, like all the way back to like the Greeks, like trying to figure out how long pregnancies are supposed to last. And again, just that being one of those things that it’s surprising how much we don’t know still about pregnancy and like all the different things that have to occur from beginning to end. So I think that was so fascinating to me. It was just like, what we know about how long pregnancies should last, and then also what we don’t know. The other thing too that I thought was really affecting was just some of the research on stillbirth as well, the different communities who are more egregiously impacted by it, as well as some of the things that people can do. Again, I think it’s something that was really important to learn and was just grateful for the opportunity to sit with that research. What’s that thing from Mr. Rogers? When you’re dealing with something hard, looking for the helpers, getting to see some of the important work that researchers are trying to do to help us better understand this, address this, and try to fight this problem in pregnancy. It was tough, but it also made me feel glad that there’s some really smart people out there putting their heads together on this and trying to find some better ways forward.

Dr. Rebecca Dekker – 00:44:06:

Agreed. I think that the connection between due dates and stillbirth can sometimes be almost either people underplay the risk. They will say things like, well, your body’s meant to go into labor when it goes into labor and you need to trust your body and trust birth. But say that to somebody who’s lost their baby, doesn’t make sense. But at the same time, we see also on the other side, we’ll see fear mongering of like, there’s a 50% chance higher your baby will die in the next week, but they don’t actually show you the absolute risk is still low. And I think every family deserves to have access to the actual numbers. And that’s one reason we put this article together so that that can be freely available. You know, it’s confusing sometimes to listen or to watch a video talking about these statistics. So if you want to just go to ebbirth.com/duedates, and we have a free one page handout for you on what is a due date. And then all of the numbers are in there from these studies about the actual risks. For me, I think we added a little section we didn’t talk about yet on the podcast about the aging of the placenta. I think you briefly mentioned it, Sara. So I think that is another thing we hear a lot is that parents are told by their providers that your placenta is getting old. Right? That has been around for decades. I hear people talk about this all the time. And I find it fascinating that we can only find two studies on the aging of the placenta. So, you know, there was a study in 2017. It was the first study we could ever find looking at biological markers of aging in placentas. And it was a small study. Often these like biological sample studies are small because they do these really complicated tests on the organ. And with this, they did find an increase in DNA and RNA damage in late birth placentas and placentas that came from stillbirths compared to placentas from 37 to 39 weeks. And, you know, that was a small study, but it does give us some interesting data to like start from. The other study was from 2019, and these were researchers from Spain, and they compared the placentas of 21 babies born at term who were typical in size to 18 babies born at term who were classified as small for gestational age. And then also they looked at more placentas from babies who were classified as having fetal growth restriction. And they found that placentas of the small for gestational age babies and the fetal growth restricted babies showed the same signs of premature aging of the placenta, which may be why those babies were not growing. Then we talk more, Sara, you added a whole section about small for gestational age. We didn’t even go into that and how that is probably one of the fetal growth restriction, I should say more specifically, is one of the biggest risk factors for stillbirth. And we talk more about that in the article.

Dr. Sara Ailshire – 00:47:04:

Yeah, absolutely. This is definitely, you know, there’s so much more that we get into with the article than we can get into here. So just, you know, if you have a chance, please go check it out. If you’re curious about anything that we’ve mentioned here, you want to know more about.

Dr. Rebecca Dekker – 00:47:16:

So the bottom line is a few things I want to summarize for you. I think we can all agree that the traditional way of calculating the estimated due date is not 100% accurate. Instead, it’s more accurate to give people a range of time in which they will probably go into labor on their own, which we talked about it being 40 weeks, five days for about half of people, if it’s their first baby, and 40 weeks, three days. And about half of people who have given birth before will give birth by 40 weeks and three days. The estimated due date can also be calculated using ultrasound, which is more accurate than using the last menstrual period. And that ultrasound accuracy is highest if it’s done between 11 and 14 weeks of pregnancy. And there are a lot of studies showing that there is an increase in the risk of stillbirth towards the end of pregnancy. Although some might consider the overall or absolute increase in risk to be small until you reach 41 weeks and later. Certain characteristics, such as it being your first pregnancy or pregnancy where the baby is growth restricted, can pose a higher risk for experiencing stillbirth in late pregnancy compared to other people. And there are actions we can take as a whole, as a population to help lower the risk of stillbirth, but some stillbirths are not preventable. And we need more research on this topic. I also want to say some people that leads us to inducing for due dates. Like when do you time an induction if you’re trying to lower the risk of stillbirth, but still give your body a chance to go into labor on its own. We’ll talk about that in the companion article, all about being induced for reaching around 41 weeks. And that article is coming out later this spring. So keep your eyes open for the podcast and the article on that subject. So Sara, thank you again for all your help. And we are so excited to launch this updated due dates article out into the world.

Dr. Sara Ailshire – 00:49:11:

Yeah. And I think we have a new handout that goes with this one.

Dr. Rebecca Dekker – 00:49:14:

Yes. The evidence on What is a Due Date is available at ebbirth.com/duedates. So check it out. Today’s podcast was brought to you by the Signature Articles at Evidence Based Birth®. Did you know that we have more than 20 peer-reviewed articles summarizing the evidence on childbirth topics available for free at evidencebasedbirth.com? It takes six to nine months on average for our research team to write an article from start to finish. And we then make those articles freely available to the public on our blog. Check out our topics ranging from advanced maternal age to circumcision, due dates, big babies, pitocin, vitamin K, and more. Our mission is to get research evidence on childbirth into the hands of families and communities around the world. Just go to evidencebasedbirth.com, click on blog. And click on the filter to look at just the EBB Signature Articles.

There are different ways of measuring stillbirth rates. Depending on how the stillbirth rate is calculated, you can end up with different rates using the same data.

Up until the 1980s, some researchers thought that the risk of stillbirth past 41-42 weeks was similar to the risk of stillbirth earlier in pregnancy. So, they did not think there was any increased risk of going past your due date.

However, in 1987, a researcher named Dr. Yudkin published a paper introducing a new way to measure stillbirth rates. Dr. Yudkin said that earlier researchers used the wrong math when they calculated stillbirth rates—they used the wrong denominator! (Yudkin et al. 1987).

Here’s why this formula is wrong: We don’t need to know how many stillbirths happen out of every 1,000 births at 41 weeks. Instead, we need to know how many stillbirths happen at 41 weeks compared to all pregnancies and births at 41 weeks. In other words, you have to include the healthy, living babies that have not been born yet in your denominator.

When researchers began using this new formula to figure out stillbirth rates, they found something very surprising—the risk of stillbirth decreased throughout pregnancy, until it reached a low point at 37-38 weeks, after which the risk started to rise again.

Even after researchers began using the new way of calculating stillbirth rates, there was still controversy about the best way to calculate this new formula for measuring stillbirth rates.

Different than what Yudkin proposed in 1987, some researchers preferred an “open-ended” stillbirth rate (also known as the “prospective risk of stillbirth”). An open-ended stillbirth rate at 40 weeks would tell us what a pregnant person’s risk of stillbirth was for any time after 40 weeks, if she let the pregnancy continue indefinitely.

Other researchers argued that most people (and doctors!) don’t want to know what the risk of stillbirth would be if a pregnant person chose to let the pregnancy continue on and on! (Hilder et al. 2000). They just want to know what the risk would be if they waited one more week until the next appointment, or even a few days.

But the “open-ended” stillbirth rate tells you what your risk of stillbirth at 40 weeks would be if you include babies born not just at 40 weeks, but 41 weeks, 42 weeks, 43 weeks, and on! (Boulvain et al. 2000).

In the end, you will find that stillbirth rates vary from study to study, depending on whether the researchers report the actual stillbirth rate, or the open-ended stillbirth rate.

Signature Articles on Related Topics:

• Evidence on: The ARRIVE Trial and Elective Induction at 39 Weeks
• Evidence on: Inducing for Due Dates

Stillbirth Prevention Organizations

• Count the Kicks
• Healthy Birth Day
• PUSH Pregnancy
• Star Legacy Foundation

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