Most babies arrive between 37 and 41 weeks, but a pregnancy can technically continue to 42 weeks or beyond before it’s classified as post-term. In practice, doctors rarely let pregnancies go past 42 weeks today, and only about 1% of births reach that point. The vast majority, roughly 83%, happen before 41 weeks.
How Pregnancy Timing Is Classified
The American College of Obstetricians and Gynecologists breaks down the final stretch of pregnancy into four categories:
- Early term: 37 weeks through 38 weeks, 6 days
- Full term: 39 weeks through 40 weeks, 6 days
- Late term: 41 weeks through 41 weeks, 6 days
- Post-term: 42 weeks and beyond
Your due date marks the end of week 40, but that date is really just a midpoint estimate. About half of all births happen before 40 weeks, and the other half happen after. Going a week or even two past your due date is common and, by itself, doesn’t mean something is wrong.
Why Due Dates Aren’t Exact
A due date is only as accurate as the method used to calculate it. First-trimester ultrasounds are the most reliable, but even those carry a margin of error of several days. By the third trimester, ultrasound dating can be off by up to three weeks in either direction. That means a pregnancy labeled “42 weeks” might actually be closer to 39 or 40 weeks, or it could genuinely be overdue.
This uncertainty is one reason providers rely on early ultrasounds to set the due date whenever possible. If your due date was established late in pregnancy, your provider may factor that uncertainty into decisions about timing.
What Happens to the Placenta After 41 Weeks
The placenta is the organ that delivers oxygen and nutrients to your baby through the umbilical cord. In most pregnancies, it functions well through 41 weeks. But in some cases, the placenta begins to deteriorate as the pregnancy stretches longer, developing areas of reduced blood flow and tissue breakdown. This is called placental insufficiency.
When the placenta can’t keep up, the baby receives less oxygen and fewer nutrients. Babies born from these pregnancies sometimes appear thin, with signs of soft-tissue wasting and depleted energy stores. The volume of amniotic fluid also tends to drop, a condition called oligohydramnios. Less fluid means the umbilical cord is more likely to get compressed during contractions, and if the baby passes meconium (its first stool) before birth, that meconium is more concentrated and more dangerous to inhale.
Placental insufficiency can technically happen at any gestational age, but it becomes most common in pregnancies that continue past 41 to 42 weeks.
How the Risks Change Week by Week
The risk of stillbirth stays relatively low and stable through weeks 39, 40, and 41, then jumps noticeably at 42 weeks. For women under 35, the stillbirth rate per 10,000 ongoing pregnancies is about 6.8 at 40 weeks, 8.5 at 41 weeks, and 28.2 at 42 weeks. That’s roughly a threefold increase between week 41 and week 42.
For women 35 and older, the numbers are higher at every stage: about 10 per 10,000 at week 40, 15.4 at week 41, and 32.5 at week 42. Maternal age amplifies the risk at each point, which is why providers often recommend earlier induction for women over 35.
The risk of neonatal death (a baby dying within the first 28 days of life) follows a similar pattern. It stays essentially the same through weeks 39, 40, and 41, then rises at 42 weeks and beyond to about 6 per 10,000. These are still small absolute numbers, but the relative jump at 42 weeks is significant enough to shape medical guidelines worldwide.
When Providers Recommend Induction
Guidelines vary by country, but they cluster around the same window. The World Health Organization recommends routine induction at 41 weeks, not before. Canadian guidelines similarly recommend induction sometime during the 41st week. ACOG’s current position is that induction between 42 and 42 weeks, 6 days is supported by strong evidence, and that induction between 41 and 42 weeks is reasonable based on more limited evidence.
The practical result is that most providers in the U.S. will discuss induction with you once you hit 41 weeks and strongly recommend it by 42 weeks. Very few pregnancies are allowed to continue past 42 weeks in countries with modern obstetric care, which is why only about 1% of births happen at or beyond that mark.
One large review found that inducing at or after 37 weeks, compared to waiting, reduced the rate of perinatal death from about 3 per 1,000 to 0.4 per 1,000. The absolute numbers are small either way, but the reduction is meaningful.
Monitoring Between 40 and 42 Weeks
If you pass your due date and choose to wait for labor to start on its own, your provider will increase how closely they monitor you and your baby. Starting around 40 to 41 weeks, this typically involves two things: a non-stress test (NST) and an ultrasound to check amniotic fluid levels.
The non-stress test tracks your baby’s heart rate for 20 to 30 minutes while you sit or recline. A healthy baby’s heart rate accelerates with movement. If the heart rate pattern looks flat or shows concerning dips, it can signal that the baby isn’t getting enough oxygen.
The ultrasound focuses on measuring the deepest pocket of amniotic fluid. If the largest pocket measures less than 3 centimeters, or if a broader index falls below 5 centimeters, that’s considered low and may prompt a conversation about delivery.
These checks are typically done every two to three days once you’re past 41 weeks. As long as fluid levels are normal and the non-stress test is reassuring, continuing to wait until 41 weeks and 6 days doesn’t appear to increase the risk of stillbirth compared to the prior week. Beyond that point, most guidelines shift firmly toward recommending delivery.
What “Overdue” Babies Look Like
Babies born post-term often look different from full-term newborns. If the placenta was functioning well the entire time, the baby may simply be larger than average, which can increase the chance of a difficult delivery, particularly shoulder complications during vaginal birth. Larger babies are also more likely to need a cesarean section.
If placental function declined, the baby may look the opposite: long and thin, with dry or peeling skin, long fingernails, and less of the white waxy coating (vernix) that protects the skin in the womb. These babies may have lower blood sugar at birth because their energy reserves were used up in the final days of pregnancy. They typically recover well with early feeding and monitoring, but they need closer attention in the first hours after delivery.