A typical pregnancy lasts 40 weeks, counted from the first day of your last menstrual period. That works out to about 280 days, or roughly nine months and one week. But that number is a starting point, not a guarantee. Healthy pregnancies routinely end anywhere from 37 to 42 weeks, and your actual delivery date depends on several biological factors that no calculator can perfectly predict.
How 40 Weeks Is Calculated
The standard 40-week timeline begins on the first day of your last menstrual period, even though conception typically happens about two weeks later, around ovulation. This means that during the first two weeks of “pregnancy” by the calendar, you aren’t actually pregnant yet. The system exists because most people can recall when their period started but not the exact day they conceived.
This method, known as Naegele’s Rule, has been used since the 1800s. It works by taking the first day of your last period, adding seven days, and counting forward nine months. It’s a reliable estimate for people with regular 28-day cycles, but it becomes less accurate if your cycles are longer, shorter, or irregular.
Ultrasound dating in the first trimester is considerably more precise. Measuring the embryo’s length between 6 and 13 weeks of pregnancy pins down the due date to within 5 to 7 days. In one study, 40% of women who received a first-trimester ultrasound had their due date adjusted because it differed from the period-based estimate by more than 5 days. If your ultrasound date and your period-based date don’t match by more than 5 days before 9 weeks, the ultrasound date is generally used instead.
Early Term, Full Term, and Beyond
Not all weeks near the due date carry equal weight. The American College of Obstetricians and Gynecologists divides the final stretch 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
- Postterm: 42 weeks and beyond
These distinctions matter because babies born even a week or two before 39 weeks have slightly higher rates of breathing problems and feeding difficulties compared to those born at full term. The labels replaced the older practice of calling everything from 37 to 42 weeks simply “term,” which made it seem like all those weeks were interchangeable.
Why Pregnancies Vary in Length
Even among healthy pregnancies with no complications, the actual length of gestation varies by several weeks from person to person. Your body doesn’t follow a rigid countdown clock. Several factors influence when labor begins naturally.
Whether this is your first pregnancy plays a role. First pregnancies tend to run slightly longer than second ones, with the average difference being small (less than a day) but the range of outcomes much wider. First-time parents also show more variability overall, meaning their deliveries are harder to predict. If you’ve had a previous preterm delivery, the effect of birth order becomes more significant, with the difference growing by up to 4 to 5 days in subsequent pregnancies.
Genetics also contributes. If your mother or sister carried pregnancies longer or shorter than average, you may follow a similar pattern. Other influences include the length of your menstrual cycle (longer cycles tend to push delivery dates later) and the timing of when the embryo actually implanted in the uterus, which can vary by several days even when the conception date is known.
What Triggers Labor
Throughout most of pregnancy, progesterone keeps the uterus relaxed and prevents contractions. Labor begins when this calming effect is gradually withdrawn. Unlike some other mammals, where progesterone levels drop sharply in the blood before delivery, humans experience a more subtle shift. The body changes how it responds to progesterone at the cellular level, reducing its quieting effect on the uterine muscle without a dramatic drop in blood levels.
As progesterone’s influence fades, estrogen activity increases. This combination triggers a cascade: the uterus becomes more sensitive to oxytocin (the hormone that drives contractions), the cervix begins to soften and thin, and inflammatory signals in the uterus ramp up. Together, these changes tip the balance from a quiet uterus to one that contracts rhythmically. The timing of this hormonal shift varies from person to person, which is a key reason pregnancies don’t all last the same number of days.
When a Pregnancy Goes Past the Due Date
Only about 5% of babies arrive on their actual due date. Going past 40 weeks is common, and a pregnancy between 40 and 41 weeks doesn’t typically require special testing. At 41 weeks, your provider will likely begin monitoring the baby more closely with tests that check heart rate patterns and amniotic fluid levels.
Reaching 42 weeks (postterm) does carry increased risks. Amniotic fluid can decrease, which may compress the umbilical cord and reduce oxygen flow to the baby. The baby may grow larger than expected, making delivery more difficult and raising the likelihood of a cesarean or assisted vaginal delivery. There is also a higher chance of the baby passing stool (meconium) before birth, which can cause serious breathing problems if inhaled. Postterm pregnancies also increase the risk of stillbirth, infection, and heavy bleeding after delivery.
For these reasons, induction of labor is typically discussed as you approach or reach 41 weeks. The exact timing depends on your individual situation, but the goal is to balance the benefits of letting labor start on its own against the rising risks of waiting too long.
Due Date Calculation After IVF
If you conceived through IVF, your due date is calculated differently because the guesswork about ovulation timing is removed. The calculation starts from the date of embryo transfer. Since there’s no need to account for the roughly two-week gap between a period and ovulation, the pregnancy timeline is about two weeks shorter on paper than the standard method. Whether you had a day-3 or day-5 embryo transfer changes the math slightly, and your fertility clinic will provide a precise due date based on the transfer date. First-trimester ultrasound is still used to confirm the timeline, just as with natural conception.