A typical pregnancy lasts 280 days, or 40 weeks, counted from the first day of your last menstrual period. That works out to roughly nine months and one week. But this number is an average, not a deadline. Healthy pregnancies routinely vary by several weeks in either direction.
Why the Count Starts Before Conception
The 40-week figure can be confusing because it doesn’t start at conception. It starts about two weeks earlier, on the first day of your last period. Doctors use this starting point because most people can identify when their period began but not the exact day they ovulated or conceived. The actual time from ovulation to birth is shorter: a median of 268 days, or about 38 weeks and 2 days.
Your due date is typically calculated by taking the first day of your last period, adding seven days, and counting forward nine months. This formula, known as Naegele’s rule, has been the standard for over a century. But it assumes a 28-day cycle with ovulation on day 14, which doesn’t apply to everyone. If your cycles are longer or shorter, your real conception date may be off by days or even weeks from what the formula assumes.
That’s one reason early ultrasounds are so useful. A first-trimester ultrasound measures the embryo’s length and can estimate gestational age to within five to seven days. In studies, about 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 five days.
How Much Pregnancy Length Varies
Even when researchers know the exact day of ovulation, pregnancy length still varies more than most people expect. A well-known study published in Human Reproduction tracked women whose conception dates were precisely identified. Among those with uncomplicated pregnancies carried to term, the spread from shortest to longest was 37 days. That’s more than five weeks of natural variation with no complications involved.
The standard deviation of human gestation is roughly nine days. In practical terms, this means that if your due date is November 15, delivering anytime from early November through late November is biologically normal. Only about 5% of babies arrive on the exact due date. Your due date is better understood as the center of a window, not a target.
What Counts as “Full Term”
Doctors no longer treat any delivery between 37 and 42 weeks as equally “term.” The categories are more specific now:
- 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
These distinctions matter because babies born at 39 weeks tend to have better outcomes than those born at 37 weeks. The final two weeks of a “term” pregnancy allow for important brain and lung development. This is why elective deliveries are generally not scheduled before 39 weeks unless there’s a medical reason.
What Happens If You Go Past Your Due Date
Going past 40 weeks is common, especially for first pregnancies. Between 40 and 41 weeks, most providers simply monitor and wait. At 41 weeks, your provider will likely recommend fetal testing to check the baby’s heart rate and fluid levels. If you reach 41 weeks without signs of labor, induction is typically recommended to reduce the small but real risks that increase as a pregnancy extends further.
Post-term pregnancy (42 weeks or later) is associated with a higher chance of complications, including reduced amniotic fluid and a larger baby that can make delivery more difficult. For this reason, very few pregnancies are allowed to continue beyond 42 weeks in modern obstetric practice.
Factors That Influence How Long You’ll Carry
Several characteristics can nudge your pregnancy shorter or longer. Whether it’s your first baby is one of the biggest factors. First-time mothers tend to carry slightly longer than those who have given birth before, sometimes by several days to a week.
Maternal age and body weight also play a role. A large population study of over 7 million births found that pre-pregnancy obesity (a BMI of 30 or higher) was associated with an 18% increased risk of preterm birth compared to a healthy-weight BMI. This association grew stronger with maternal age: for women 35 and older, the link between higher BMI and earlier delivery was more pronounced than for younger women. These patterns held across racial and ethnic groups, though the exact numbers differed.
Other influences include the length of your menstrual cycle (longer cycles tend to correlate with slightly longer pregnancies), genetics, and whether you’ve had a previous preterm birth. None of these factors override the general 37-to-41-week range for most healthy pregnancies, but they help explain why your experience may differ from a friend’s or a sibling’s.
The Short Answer
Pregnancy is dated at 40 weeks from your last period, but the biological reality from conception to birth is closer to 38 weeks. A healthy delivery anytime between 39 and 41 weeks is considered ideal, and natural variation of two to three weeks in either direction is completely normal. Your due date is an estimate, not an expiration date.