A normal pregnancy lasts 40 weeks, or 280 days, counted from the first day of your last menstrual period. That said, only about 5% of babies actually arrive on their estimated due date. Most healthy pregnancies end somewhere between 39 and 41 weeks, and that entire window is considered normal.
How the 40-Week Number Is Calculated
Your due date is typically set using a formula that takes the first day of your last menstrual period and adds 280 days. This method has been the standard for over a century, and it works reasonably well, though it slightly overestimates pregnancy length for most women. One study found the average difference between the formula’s prediction and actual delivery was about 3.5 days, with 60% of women delivering before their predicted date.
The 40-week count starts about two weeks before you actually conceived, since ovulation usually happens around day 14 of your cycle. So the biological pregnancy, from fertilization to birth, is closer to 38 weeks. But because the date of conception is rarely known with certainty, providers stick with the last-period method as a starting point.
If you have an ultrasound in the first trimester, it can pin down your due date to within 5 to 7 days. That’s the most accurate window for dating a pregnancy. Later ultrasounds become less precise because babies grow at increasingly different rates.
What “Full Term” Actually Means
Not all weeks near the end of pregnancy carry equal weight. Medical organizations now break down the final stretch into 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
These distinctions matter because babies born even a week or two early can have different outcomes than those born at 39 weeks or later. The “full term” label was narrowed to the 39-to-40-week window specifically to discourage elective early deliveries that don’t have a medical reason behind them.
First Pregnancies Tend to Run Longer
If this is your first baby, expect the possibility of a longer wait. First-time mothers carry for an average of 275.9 days, compared to 274.5 days for women who have delivered before. That difference of about a day and a half is modest on its own, but it shows up more dramatically at the tail end of pregnancy. First-time mothers are 1.53 times more likely to go past 41 weeks: 6.2% of first deliveries extend beyond that point, compared to only 4.0% for women who have given birth previously.
When researchers tracked pregnancies where labor started on its own without medical intervention, the average gestational age at delivery was just over 280 days regardless of whether it was a first or subsequent birth. The natural variation around that average, though, spans roughly a week in either direction.
Why Labor Starts When It Does
The exact trigger for labor remains one of the less understood processes in human biology, but researchers at Stanford identified a cascade of changes in maternal blood in the weeks before delivery. Steroid hormones like progesterone and cortisol surge. Factors that support blood vessel growth start to decline, which likely begins loosening the connection between the placenta and the uterine wall. At the same time, blood clotting factors ramp up, preparing the body to prevent excessive bleeding during delivery.
There’s also an immune component. Near the end of pregnancy, fragments of placental material and fetal cells increasingly enter the mother’s bloodstream, which can provoke an immune response. One key signal researchers identified was a regulatory immune protein that suppresses inflammation. Changes in this protein were among the strongest predictors that labor was approaching. All of these shifts happen gradually over the final two to three weeks, which is part of why pinpointing an exact delivery date is so difficult.
What Happens If Pregnancy Goes Past 41 Weeks
Going past your due date is common and, in most cases, not immediately dangerous. But the risks do increase as a pregnancy moves into late-term and post-term territory. These include a higher chance of stillbirth, the baby growing unusually large (which complicates delivery), and decreased amniotic fluid. Lower fluid levels can allow the umbilical cord to become compressed, restricting oxygen flow to the baby. There’s also a greater risk that the baby will inhale meconium (their first stool), which can cause serious breathing problems after birth.
For the mother, post-term pregnancies carry higher rates of assisted vaginal delivery, cesarean delivery, infection, and postpartum hemorrhage. Problems occur in only a small number of post-term pregnancies overall, but most providers will begin monitoring you more closely after 41 weeks and discuss the option of inducing labor rather than waiting for it to begin on its own.
Why Your Due Date Is a Best Guess
Given all the biological variability involved, it helps to think of your due date as the center of a window rather than a deadline. The standard deviation for delivery timing is close to 7 days, meaning roughly two-thirds of spontaneous deliveries happen within a week of the 280-day mark. The remaining third fall further out in either direction. Your individual timing depends on factors including your cycle length, whether you’ve given birth before, your age, and genetic variation that researchers are still working to understand.
If your cycles are longer or shorter than the standard 28 days, your actual ovulation date may shift your true due date by several days compared to what the formula predicts. A first-trimester ultrasound can correct for this, which is one reason early prenatal care improves the accuracy of delivery estimates. Even so, a margin of error of 5 to 7 days is built into the best available dating methods. Planning for a range of 39 to 41 weeks, rather than a single circled date on the calendar, is the most realistic approach.