A typical pregnancy lasts about 40 weeks, or 280 days, counted from the first day of your last menstrual period. That’s roughly nine calendar months, though the actual time between conception and birth is closer to 38 weeks since most people ovulate about two weeks into their cycle. Your due date is an estimate, not a deadline, and healthy babies routinely arrive anywhere from a few weeks before to a week or so after that target.
How Due Dates Are Calculated
The traditional method adds 280 days to the first day of your last period. This assumes a textbook 28-day cycle with ovulation on day 14, which doesn’t reflect reality for many people. Only about half of women accurately recall the date of their last period, and cycle lengths vary widely. That’s why early ultrasound has become the preferred way to pin down a due date.
An ultrasound during the first trimester (up to about 14 weeks) is the most accurate dating tool available. In one study, 40% of women who received a first-trimester ultrasound had their due date shifted by more than five days compared to the date calculated from their last period. Even when you’re confident about your cycle dates, an early scan can sharpen the estimate. Second-trimester ultrasounds are less precise, leading to due date changes in only about 10% of cases.
What “Full Term” Actually Means
For years, any birth between 37 and 42 weeks was simply called “term.” That five-week window turned out to be misleadingly broad, because babies born at 37 weeks face meaningfully different health outcomes than those born at 39 weeks. In 2013, the American College of Obstetricians and Gynecologists and the National Institutes of Health introduced more specific 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
- Post-term: 42 weeks and beyond
The sweet spot for delivery, in terms of fetal development and lowest risk of complications, falls in that full-term window of 39 to 40 weeks. Babies born in the early-term range are generally healthy, but they have slightly higher rates of breathing difficulties and feeding problems compared to those who make it to 39 weeks.
Most First-Time Mothers Go Past Their Due Date
If you’re a first-time parent watching the calendar tick past 40 weeks, you’re in good company. A large study of over 12,000 low-risk first-time mothers found that the median point for spontaneous labor was 40.1 weeks. More telling, 37% of these women went into labor during the week after their due date. Going a few days or even a full week past your estimated date is not just common for first pregnancies, it’s the statistical norm.
Subsequent pregnancies tend to be slightly shorter, though not always. Individual variation is significant, and no formula reliably predicts exactly when your labor will start.
What Triggers Labor
The onset of labor involves a coordinated hormonal conversation between the fetus and the mother’s body. As the baby’s brain and adrenal glands mature in the final weeks, the fetus begins producing rising levels of the stress hormone cortisol. This cortisol serves a dual purpose: it helps the baby’s lungs and other organs finish developing, and it triggers a chemical shift in the placenta that lowers the mother’s levels of progesterone, the hormone that has been keeping the uterus relaxed throughout pregnancy.
As progesterone drops, other signals ramp up. The uterus becomes more sensitive to oxytocin, and the cervix begins to soften as structural proteins in its tissue break down. The membranes surrounding the baby also play a role. As they age, they release inflammatory signals into the amniotic fluid that appear to help push the process along. The result is a cascade: the cervix opens, contractions strengthen and become rhythmic, and labor begins.
Factors That Influence Pregnancy Length
Several maternal characteristics can shift pregnancy duration in either direction. Body weight before pregnancy is one of the most studied. Pre-pregnancy obesity (a BMI of 30 or higher) is associated with about an 18% increased risk of preterm birth, meaning delivery before 37 weeks. That risk climbs with maternal age. For women 30 and older, the combination of obesity and advancing age raises preterm birth risk more steeply than either factor alone.
The relationship between weight and preterm risk also varies by race and ethnicity. In population-level data, the age at which obesity begins to increase preterm birth risk differs: around age 20 for white and Hispanic women, and around age 30 for Black women. These patterns likely reflect a complex mix of biological, environmental, and systemic health factors rather than any single cause.
Risks of Going Past 42 Weeks
A pregnancy that stretches past 42 weeks is classified as post-term, and while most post-term pregnancies still end with healthy deliveries, the risks do rise. The placenta, which has been sustaining the baby for nine months, gradually becomes less efficient. Amniotic fluid levels can drop, increasing the chance that the umbilical cord gets compressed and reduces oxygen flow. The baby may continue to grow larger, which raises the likelihood of a difficult delivery, assisted vaginal birth, or cesarean section.
Other post-term risks include stillbirth, the baby inhaling meconium (its first stool) into the lungs before delivery, and higher rates of infection and heavy bleeding for the mother after birth. These complications remain uncommon in absolute terms, but they’re the reason most providers will discuss induction of labor as a pregnancy approaches or passes 41 weeks.
Why Your Due Date Is a Best Guess
Even with an accurate early ultrasound, a due date is a midpoint of probability, not a precise prediction. The biological signals that start labor depend on fetal development, hormonal timing, and factors unique to each pregnancy. Two pregnancies in the same person can differ by weeks. Expecting delivery on exactly one date sets up unnecessary anxiety. A more realistic frame is a “due window” spanning roughly 37 to 41 weeks, with the highest probability clustered right around 40 weeks and the days just after.