A full term pregnancy lasts 39 weeks through 40 weeks and 6 days, as defined by the American College of Obstetricians and Gynecologists (ACOG). That translates to about 280 days counted from the first day of your last menstrual period, or roughly 268 days (38 weeks and 2 days) from the actual moment of conception. But these numbers are averages, and the natural variation is wider than most people expect.
The Four Categories of “Term” Pregnancy
For decades, any pregnancy between 37 and 42 weeks was simply called “term,” as if those five weeks were interchangeable. Medical organizations changed that in 2013, splitting the window into four distinct categories because outcomes for babies differ significantly across those weeks:
- 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
- Post-term: 42 weeks and beyond
The distinction matters because babies born at 39 weeks or later have the best chance at healthy outcomes compared with babies born before that threshold. Those final one to two weeks involve meaningful physical development, not just weight gain.
What Happens in Those Last Weeks
At 37 weeks, a baby can grasp firmly and the head may begin dropping into the pelvis, but the lungs, brain, and liver are still maturing. The brain at 35 weeks weighs roughly two-thirds of what it will weigh at 39 to 40 weeks, so the final stretch involves rapid neural growth.
By 38 weeks, the fine hair covering the body (lanugo) has mostly disappeared, toenails have reached the tips of the toes, and the head and belly circumference are roughly equal. At 39 weeks, the chest is expanding, and fat is being added across the body to help regulate temperature after birth. This fat layer is one reason full term babies tend to maintain body heat and blood sugar more effectively than early term babies.
How Your Due Date Is Calculated
The standard method counts 280 days (40 weeks) from the first day of your last menstrual period. This formula, known as Naegele’s rule, assumes a 28-day cycle with ovulation on day 14. In reality, cycle lengths vary, ovulation timing shifts, and roughly half of women don’t accurately recall the exact start of their last period.
That’s why first-trimester ultrasound has become the gold standard for dating a pregnancy. In one study, 40% of women who received a first-trimester ultrasound had their due date adjusted because the scan disagreed with their menstrual dating by more than 5 days. Before 9 weeks, even a 5-day discrepancy between the ultrasound measurement and the period-based estimate is enough to change the official due date. Second-trimester ultrasound is less precise, only prompting a change about 10% of the time.
How Much Pregnancy Length Varies
Even with accurate dating, natural pregnancy length varies by as much as five weeks from one healthy woman to another. A large study tracking pregnancies from the confirmed date of ovulation found the average time from ovulation to birth was 268 days, but the range was wide. Only 4% of women deliver on their estimated due date, and just 70% deliver within 10 days of it, even when the date was set using ultrasound.
Factors like age, prior pregnancy history, and even the length of time it took the embryo to implant can shift the timeline. This variability is normal and doesn’t necessarily signal a problem in either direction.
Why Going Past 41 Weeks Raises Concerns
Most pregnancies that stretch past the due date still end well, but the risk of complications does climb with each passing week. The risk of stillbirth rises steadily after 40 weeks and peaks after 43 weeks, where it reaches more than 10 times the rate seen in the weeks before the due date. A large national study found that when hospitals began offering induction around 41 weeks rather than waiting longer, the risk of fetal death in that week dropped by 30 to 38%.
This is why most providers will begin closer monitoring if you pass 41 weeks and will typically recommend induction before 42 weeks. The conversation usually starts around your due date if labor hasn’t begun on its own.
Elective Induction at 39 Weeks
For healthy first-time mothers carrying a single baby, research now suggests that induction at exactly 39 weeks may slightly reduce the chance of needing a cesarean delivery. This was a shift from older thinking, which held that induction itself increased surgical risk. The option isn’t appropriate for every pregnancy, but it’s increasingly part of the conversation for low-risk first pregnancies that have reached full term.
When your provider discusses timing, they’ll assess cervical readiness using a scoring system that evaluates how dilated, thinned, soft, and positioned your cervix is, along with how far the baby’s head has descended into the pelvis. A higher score means your body is already moving toward labor and induction is more likely to proceed smoothly.
The Short Answer
Full term is 39 weeks 0 days through 40 weeks 6 days. That’s the window associated with the best outcomes for both mother and baby. Most pregnancies fall somewhere between 37 and 42 weeks, with enormous natural variation even among perfectly healthy women. Your due date is a midpoint estimate, not a deadline.