You will not be pregnant forever. I remember that at some point during the third trimester, I started to panic a bit. What if I don’t go into labor by her due date? What if I’m still pregnant at 42 weeks? What if she never comes out? (At this point, I could only really waddle around my house, so 42 weeks felt like an eternity.)
Some of this worry is reasonable. Many women do go past their due date — especially with a first child, and especially without interventions like induction. With my first pregnancy, I wasn’t planning to be induced, and I was happy (well, resigned) to wait until my due date.
Even so, I wanted to know how likely I was to give birth before then. What is the chance of labor in week 37 vs. week 38? This felt like essential information for not only my sanity but also my family’s planning. What week should we have the hospital bag packed? What week should my husband and I have all the work coverage we’ll need, just in case?
Here’s what I found.
What week are you most likely to give birth?
First, let’s look at all singleton births in the U.S. (we’ll get to twins below). On average, you are most likely to have your baby in the 39th week of pregnancy. In fact, about 70% of all babies are born before their due date.
This graph is nice for seeing a general picture of the data. But if you’re still pregnant on your due date, it’s not that helpful to know you had a 70% chance of already giving birth. If you reorganize the data a bit, you can get a clearer picture of what to expect each week:
This view of the data tells us that if you get to your due date without a baby, there is a 60% chance you’ll have the baby in the next seven days. If you haven’t had the baby by 41 weeks, there is about a 60% chance you’ll go into labor spontaneously. At 42 weeks, the vast majority of doctors will induce labor. Keep in mind these are averages, but they can still be informative (and comforting) to see.
I know that many of you reading this are going into your first birth, and some of you may have twins coming. So let’s dive even deeper into this data to get you a more specific window.
What about first births?
If this is your first time giving birth, you are still most likely to have your baby in the 39th week of pregnancy, whether it’s spontaneous or an induction. (This makes sense, given the push toward elective inductions at this time.)
For spontaneous births, about 22% of them happen in week 40. After that, the share of spontaneous births drops below 10%.
What to take from this? You’ll definitely want to have your hospital bag packed by week 38 — and for a smaller number of you, you may find you need it even sooner.
What about twins?
When it comes to twins, the data looks quite different. That’s because most patients with twins do not make it past 38 weeks due to spontaneous labor, ruptured membranes, or some other compelling reason to deliver.
The majority of spontaneous births with twins happen in week 36 or 37. Inductions are most common in weeks 37 and 38. It’s exceedingly rare for any births beyond week 39.
This is a case where you will want to have conversations with your doctor about the risks and possible outcomes.
What is a full-term pregnancy?
Historically, “full term” was defined as birth at 37 weeks or later. This definition wasn’t really intended to have any particular biological meaning. There isn’t any sharp change in fetal maturity at 37 weeks and 0 days versus 36 weeks and 6 days. But when we are reporting population statistics like “share of babies preterm,” we need to have some way to define that. At or after 37 weeks, most babies do very well, so that was the timing that was picked.
However, over time, people came to realize that babies born at 39 or 40 weeks were doing, on average, better than those born at 37 or 38 weeks. They were less likely to need time in the NICU, had higher Apgar scores, and generally needed fewer other supports. Both groups are low-risk, but it is clear in the data that there is a difference even within this post-37-week period.
This wouldn’t necessarily pose a problem for the definition, but saying that after 37 weeks was “full term” made it unclear to some providers that 37 weeks was different from 39 weeks. Some providers — in some cases pushed by patients — were inducing labor at 37 or 38 weeks. This leads to worse outcomes, on average, than waiting until 39 weeks.
In light of the new data on differences in outcome by weeks, and with this concern about inductions, in 2013, national authorities suggested a redefinition into “early term” (37 to 38 weeks) and full term (39 to 40 weeks). The result has been a decline in the use of elective inductions before 39 weeks, which is a positive change for infant health.
It remains true that if you go into spontaneous labor after 37 weeks, generally babies do great, but unless there is a medical reason, your doctor is unlikely to induce at this point.
What happens if my baby is born preterm?
About 10% of deliveries are preterm (before 37 weeks). There is vast variation within the group of preterm babies. Babies born as early as 23 weeks of gestation can survive. But the experience of having a child born at 36 weeks and one born at 24 weeks is likely to be completely different.
Prebirth can cause developmental disabilities, both physical and intellectual. These differences tend to be quite small, particularly for late-preterm infants (born between 34 and 36 weeks). Prematurity is also a leading cause of infant mortality in the U.S. While this may sound scary, the vast majority of preterm infants survive. In fact, by 36 weeks, survival is 99% (compared with 99.5% at full term). And mortality rates for extremely preterm infants are declining dramatically over time.
There are a number of options for pregnant women with a prior preterm birth or additional complications (such as preeclampsia), so ask your doctor about your risk level and how to manage it if you have any concerns.
The bottom line
- Most singleton babies are born during the 39th week of pregnancy, with about 70% of births occurring before the due date. If you reach your due date, there’s a 60% chance of giving birth within the next week.
- First-time mothers are most likely to give birth in the 39th or 40th week. For twins, spontaneous births typically occur around weeks 36 and 37, and few pregnancies go beyond 38 weeks, due to medical considerations.
- “Full term” is now defined as 39 to 40 weeks, while 37 to 38 weeks is called “early term.”
- About 10% of babies are born preterm, but outcomes vary widely by gestational age.
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