A post-term pregnancy is one that extends to 42 weeks of gestation or beyond. While a typical pregnancy lasts about 40 weeks from the first day of the last menstrual period, the American College of Obstetricians and Gynecologists (ACOG) considers anything from 39 through 40 weeks and 6 days “full term.” Once a pregnancy reaches 42 weeks and 0 days, it officially crosses into post-term territory, and the risks to both mother and baby begin to rise significantly.
How Post-Term Differs From Late-Term
The distinction matters more than it might seem. ACOG breaks down the final weeks of pregnancy into four categories: early term (37 weeks through 38 weeks and 6 days), full term (39 weeks through 40 weeks and 6 days), late term (41 weeks through 41 weeks and 6 days), and post-term (42 weeks and beyond). A pregnancy at 41 weeks is late-term, not post-term, though risks are already climbing at that point. The perinatal death rate at 42 weeks is roughly twice that at full term (4 to 7 per 1,000 deliveries compared to 2 to 3 per 1,000). By 43 weeks, that rate quadruples. By 44 weeks, it increases five to sevenfold.
Why Some Pregnancies Go Past the Due Date
Multiple factors can push a pregnancy past 42 weeks. Genetics play a role: if your mother or sister had post-term pregnancies, your chances are higher. A previous post-term pregnancy is one of the strongest predictors. Maternal obesity and higher body mass index during pregnancy are also associated with going overdue. Being a first-time mother has traditionally been considered a risk factor, though some newer research from large population studies has shown mixed results on that front.
One important and often overlooked cause of a “post-term” diagnosis is simply getting the due date wrong. Only about half of women accurately recall the first day of their last menstrual period, and the standard calculation assumes a regular 28-day cycle with ovulation on day 14. That doesn’t reflect reality for many women. In one study, 40% of women who received a first-trimester ultrasound had their due date adjusted by more than five days. First-trimester ultrasound dating substantially reduces the number of pregnancies that get mislabeled as post-term, which in turn reduces unnecessary inductions.
What Happens to the Placenta
The placenta has a functional lifespan. As pregnancy extends past its due date, the placenta’s ability to deliver oxygen and nutrients to the baby gradually declines. This is sometimes called placental insufficiency. The placenta can develop areas of calcification and reduced blood flow, making it progressively less effective at supporting the baby. When the placenta can no longer keep up, the baby may experience oxygen deprivation, slowed growth, or stress responses like releasing meconium (the first bowel movement) into the amniotic fluid before birth.
Risks for the Baby
Post-term babies face several specific complications. The stillbirth risk during the 41st week is estimated at 1.04 to 1.27 per 1,000 ongoing pregnancies. At or beyond 42 weeks, that rises to 1.55 to 3.1 per 1,000. While these numbers are still relatively small in absolute terms, the upward trend is steep and continues with each additional week.
Macrosomia, or an unusually large baby, occurs in 2.5 to 10% of post-term pregnancies, compared to just 0.8 to 1% of full-term deliveries. Babies weighing 4.5 kilograms (about 10 pounds) or more are at higher risk for a difficult delivery, including shoulder dystocia, where the baby’s shoulder gets stuck behind the mother’s pelvic bone during birth.
Meconium aspiration syndrome is another concern. When a stressed baby passes meconium in utero and then inhales it with their first breaths, it can cause serious respiratory problems, including rapid breathing, low oxygen levels, and reduced lung function. This complication occurs at significantly higher rates in post-term newborns. On the other end of the spectrum, some post-term babies actually lose weight in the womb due to the failing placenta, a condition called dysmaturity, leaving them thin and fragile at birth.
Risks for the Mother
The risks don’t fall on the baby alone. At 41 weeks, the rate of primary cesarean delivery jumps sharply, from about 9% at 40 weeks to 14% at 41 weeks. Postpartum hemorrhage also becomes more likely. Severe perineal tearing (third or fourth-degree lacerations) increases with each week past 39 weeks. These maternal complications are largely driven by the baby’s increasing size and the greater likelihood of needing labor interventions as the pregnancy progresses.
How Post-Term Pregnancy Is Monitored
Once a pregnancy passes 41 weeks, the focus shifts to close surveillance of the baby’s well-being. The two most common tools are the nonstress test and the biophysical profile. A nonstress test tracks the baby’s heart rate in response to its own movements, looking for healthy patterns of acceleration. A biophysical profile goes further, using ultrasound to evaluate five areas: heart rate, breathing movements, body movements, muscle tone, and the volume of amniotic fluid surrounding the baby. A modified version combines the nonstress test with just the amniotic fluid measurement.
These tests are typically performed once or twice per week, depending on the results. A declining volume of amniotic fluid or a nonreassuring heart rate pattern often prompts a conversation about inducing labor rather than continuing to wait.
Induction and Management
Guidelines vary slightly by country, but the general consensus is that labor should be induced before a pregnancy reaches 42 weeks. ACOG recommends offering induction or continued monitoring starting at 41 weeks. British guidelines recommend induction between 41 and 42 weeks for uncomplicated pregnancies. German guidelines suggest offering induction after 41 weeks and recommending it by 41 weeks and 3 days.
Before formal induction, many providers will offer membrane sweeping as a less invasive first step. During a vaginal exam, the provider separates the membranes from the lower part of the uterus with a circular finger motion. This triggers the body to release natural prostaglandins, which help soften the cervix and can kick-start contractions. One study found that membrane sweeping successfully initiated labor and vaginal delivery in about 86% of women at 40 or more weeks. Most of those women needed only one or two sweeps, and complication rates were low for both mothers and babies. Starting membrane sweeping at 41 weeks has been shown to reduce the number of pregnancies that reach the post-term threshold without increasing negative outcomes for newborns.
The Role of Accurate Dating
Getting the due date right is one of the simplest ways to avoid the cascade of worry and intervention that comes with a post-term label. A first-trimester ultrasound, ideally performed before 14 weeks, provides the most accurate estimate of gestational age. When researchers compared groups of women who received early versus later ultrasounds, the early-ultrasound group needed fewer post-term inductions. A Cochrane review confirmed that routine early ultrasound reduces the rate of post-term diagnoses and catches multiple pregnancies sooner. If you haven’t had a first-trimester ultrasound and your pregnancy appears to be going past its due date, the actual gestational age may be less advanced than the calendar suggests.