The risks of some complications go up as you go past your due date, and there are three recent studies that have shown us what the risks are.
- In 2003, Caughey et al. looked at 135,560 women who gave birth at term in California between the years 1995 and 1999 (Caughey et al. 2003). The women in this sample all gave birth at Kaiser Permanente hospitals in northern California. The overall use of interventions (C-sections and inductions) in this sample was not listed.
- In 2004, Caughey et al. looked at the records of 45,673 women who gave birth in a single hospital in California from 1992 to 2002 (Caughey & Musci 2004). The women in this study were mostly well-educated. As far as intervention rates go, 18% gave birth by Cesarean and 16% with the help of vacuum or forceps. The rate of inductions was not listed.
- In 2007, Caughey et al. studied the medical records of 119,254 women who gave birth after 37 weeks at Kaiser Permanente between the years of 1995 and 1999. This was the same time period and same hospital as his 2003 study, but this time the researchers only looked at low-risk women who had health insurance. The overall C-section rate was 13.8%, and 9.3% gave birth with the help of vacuum or forceps. The authors also took whether or not women had inductions into account when they calculated the risks of going past your due date (Caughey 2007).
Risks for mothers:
- The risk of chorioamnionitis (infection of the membranes) was lowest at 37 weeks (0.16%) and increased every week after that to a high of 6.15% at ≥ 42 weeks (Caughey et al. 2003)
- The risk of endomyometritis (infection of the uterus) was lowest at 38 weeks (0.64%) and increased every week after that to a high of 2.2% at ≥ 42 weeks (Caughey & Musci 2004)
- The risk of having a placenta abruption (placenta separates prematurely from the uterus) was lowest at 37 weeks (0.09%), and increased every week to a high of 0.44% at ≥ 42 weeks (Caughey et al. 2003)
- The risk of preeclampsia was lowest at 37 weeks (0.4%) and highest at 40 weeks (1.5%), after which the risk did not change (Caughey et al. 2003)
- The risk of postpartum hemorrhage was lowest at 37 weeks (1.1%) and increased almost every week to a high of 5% at 42 weeks (Caughey 2007)
- The risk of a primary Cesarean (in women who have never had a Cesarean before) increased from 14.2% at 39 weeks to a high of 25% at ≥42 weeks (Caughey & Musci 2004)
- The risk of having a primary Cesarean for a non-reassuring fetal heart rate was lowest at 37-39 weeks (13.3-14.5%) and reached a high of 27.5% at 42 weeks (Caughey 2007)
- The risk of needing forceps or vacuum assistance increased from 14.1% at 38 weeks to a high of 18.5% at 41 weeks (Caughey & Musci 2004)
- The risk of having a 3rd or 4th degree tear was lowest at 37 weeks (3.4%) and increased every week to a high of 9.1% at 42 weeks. However, these numbers are much higher than are typically seen, and are partially related to the high use of vacuum and forceps in this study.
Rebecca went in to labor at 41 weeks expecting a home birth, but soon after labor began she noticed that her blood pressure was high and she had a headache. Rebecca went to the hospital with her midwife to be evaluated, and found that she had developed severe preeclampsia with neurological involvement. Rebecca agreed to Pitocin, an epidural, and magnesium. She gave birth vaginally to her son at the hospital.
In their 2007 study, Caughey et al. reported that high use of induction, Cesareans, and vacuum/forceps for women with increasing gestational age may contribute to an increase in maternal risks. However, when the researchers used a statistical method to control for the use of interventions, the risks still increased with gestational age.
Risks for babies:
- The risk of moderate or thick meconium increased every week starting at 38 weeks, and peaked at ≥42 weeks (3% at 37 weeks, 5% at 38 weeks, 8% at 39 weeks, 13% at 40 weeks, 17% at 41 weeks, and 18% at >42 weeks) (Caughey & Musci 2004)
- Neonatal intensive care unit (NICU) admission rates were lowest at 39 weeks (3.9%) and rose to 5% at 40 weeks and 7.2% at ≥42 weeks (Caughey & Musci 2004)
- The risk of the baby being large at birth (>9 lbs 15 oz or >4500 grams) rose starting at 38 weeks (0.5%), and doubled every week after that up until 42 weeks (6%) (Caughey & Musci 2004)
- The odds of having a low 5-minute Apgar score went up starting at 40 weeks and increased each week until ≥42 weeks (exact numbers not reported; Caughey & Musci 2004)
Other risks for post-term pregnancy include having low fluid, and something called dysmaturity syndrome (growth restriction plus muscle wasting), which happens in about 10% of babies who go past 42 weeks. For more information about meconium, see this article by Midwife Thinking about meconium stained waters.
What about the risk of stillbirth?
In this section, we will talk about how the risk of stillbirth increases towards the end of pregnancy. There are two very important things for you to understand when learning about stillbirth rates in post-term pregnancies. First, there is a difference between absolute risk and relative risk. Absolute risk is the actual risk of something happening to you. For example, if the absolute risk of having a stillbirth at 41 weeks was 0.4 out of 1,000, then that means that 0.4 women out of 1,000 (or 4 out of 10,000) will experience a stillbirth. Relative risk is the risk of something happening to you in comparison to somebody else. If someone said that the risk of having a stillbirth at 42 weeks compared to 41 weeks is 50% higher, then that sounds like a lot. But the actual (or absolute) risk would still be low—0.6 per 1,000 versus 0.4 per 1,000. Yes—0.6 is 50% higher than 0.4, if you do the math! So, while it is a true statement to say “the risk of stillbirth increases by 50%,” it can be a little misleading if you are not looking at the actual numbers behind it. The second important thing that you need understand is that there are different ways of measuring stillbirth rates. Depending on how the rate is calculated, you can end up with different rates.
How do you measure stillbirth rates?
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 increase in risk with 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, Wood 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. This finding—that the risk of stillbirth decreases throughout pregnancy, and then increases sometime after 37-38 weeks—has been found many times by different researchers in different countries. This phenomenon is called the “U-shaped curve” of stillbirth. In other words, there are higher rates of stillbirth earlier in pregnancy, then they go down around 37-38 weeks, after which they rise again. Because the risk of stillbirth starts to go up even more at 40, 41, and 42 weeks, some researchers argue that although 40 weeks and 3-5 days may be the physiological length of pregnancy, 40 weeks may be the functional length of a pregnancy. In other words, the average pregnancy normally lasts about 40 weeks and 5 days, but in some researchers’ opinion, because of the increased risk of stillbirth and newborn death; 40 weeks may be as long as a pregnancy should go. And although the stillbirth rates may seem really low overall, if you happen to be a parent who experiences the 1 in 1,000 event at 42 weeks, then the risk doesn’t seem so low anymore.
Actual stillbirth rates vs. open-ended stillbirth rates
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 woman’s risk of stillbirth was for any time after 40 weeks, if she let the pregnancy continue indefinitely. Other researchers argued that most women (and doctors!) don’t want to know what the risk of stillbirth would be if a woman 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.
So what is the risk of stillbirth as you go past your due date?
Since the late 1980’s, there have been at least 12 large studies that looked at the risk of stillbirth during each week of pregnancy. Some of the researchers used open-ended stillbirth rates, and some of them used actual stillbirth rates. All of the researchers found a relative increase in the risk of stillbirth as pregnancy advanced. To get an accurate picture of stillbirth in women who go past their due date, it would be best to look at studies that took place in more recent times. I’ve chosen 3 of the most recent studies to show you from Norway, Germany, and the U.S. To see all of the other studies, click to view the entire table (Table is undergoing final formatting and will be COMING SOON!! Make sure to order the free PDF so that you can receive the table by email) All 3 of these studies used the actual stillbirth rate—not the open-ended stillbirth rate. Two studies used ultrasound to calculate gestational age, and one study used the LMP.