A typical pregnancy lasts about 40 weeks, counted from the first day of your last menstrual period. That works out to roughly 280 days, or just over nine calendar months. Most babies arrive somewhere between 37 and 42 weeks, and the exact timing varies more than many people realize.
How the 40-Week Count Works
Pregnancy is dated from the first day of your last menstrual period, not from the day you actually conceived. Since ovulation and conception typically happen about two weeks into your cycle, the baby is really developing for around 38 weeks. A study tracking pregnancies from the confirmed date of ovulation found the average time from ovulation to birth was 268 days, which is 38 weeks and two days.
The 40-week estimate assumes a textbook 28-day menstrual cycle with ovulation on day 14. In reality, cycles vary. Only about half of women accurately recall the date of their last period, and even when they do, irregular cycles can throw the math off. That’s why an early ultrasound is considered the most reliable way to pin down how far along you are. Measurements taken in the first trimester are accurate to within five to seven days. In one study, 40% of women who had a first-trimester ultrasound had their due date changed by more than five days compared to what the period-based calculation predicted.
What “Full Term” Actually Means
Not all deliveries between 37 and 42 weeks are considered the same. The National Institutes of Health and the American College of Obstetricians and Gynecologists break it down into four 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 categories exist because babies born at 39 or 40 weeks tend to have fewer health problems than those born at 37 weeks, even though 37 weeks was once considered “term.” The distinction matters if your provider discusses scheduling a delivery. Waiting until at least 39 weeks, when there’s no medical reason to deliver sooner, gives the baby’s brain, lungs, and liver more time to mature.
Natural Variation in Pregnancy Length
Even among healthy pregnancies with no complications, the length of gestation can vary by as much as five weeks. Research from the European Society of Human Reproduction and Embryology found this range held true even after accounting for differences in when ovulation occurred. In other words, the variation isn’t just a measurement error. Some pregnancies are genuinely shorter or longer than others.
Several factors influence where you fall in that range. Women with a higher body mass index are more likely to carry past their due date. First-time mothers also tend to have slightly longer pregnancies compared to women who have given birth before. Older mothers, particularly first-time mothers in their early 30s, may experience longer pregnancies as well, though the effect of age is modest.
Twin and Multiple Pregnancies Are Shorter
If you’re carrying more than one baby, the timeline changes significantly. Most twin pregnancies deliver at around 36 weeks, with a typical range of 32 to 38 weeks depending on whether the twins share a placenta. A little more than half of twin pregnancies end before 37 weeks, which is technically preterm. This isn’t necessarily a sign that something went wrong. The uterus simply reaches its physical limits sooner with two or more babies, and the placenta may not sustain multiples as long as it does a single pregnancy. The interventions that help prevent preterm labor in singleton pregnancies are less effective with multiples.
What Happens if You Go Past 42 Weeks
Going past your due date is common. Going past 42 weeks is less so, and it carries real risks. The placenta, which has been your baby’s life support system, becomes less efficient the longer a pregnancy continues. Amniotic fluid levels can drop, which increases the chance that the umbilical cord gets compressed and restricts oxygen flow. The baby may also pass meconium (its first bowel movement) while still in the womb, and inhaling that substance into the lungs can cause serious breathing problems after birth.
Post-term pregnancies also raise the risk of the baby growing unusually large, which makes delivery more difficult and increases the likelihood of a cesarean section or assisted vaginal delivery. For the mother, there’s a higher chance of infection and heavy bleeding after delivery. For these reasons, most providers will discuss inducing labor if you reach 41 to 42 weeks without signs of labor starting on its own.
Why Your Due Date Is an Estimate
Only about 4 to 5% of babies arrive on their actual due date. Think of it less as a deadline and more as the center of a window. Most healthy babies arrive within two weeks on either side of that date. Your provider may adjust the window based on your ultrasound measurements, your health, and whether you’re carrying multiples. The 40-week number is a useful benchmark, but the real answer to “how long will I carry this baby” is almost always “somewhere close to 40 weeks, give or take.”