Pregnancy lasts about 9 months, or more precisely, 40 weeks (280 days) from the first day of your last menstrual period. That works out to just over 9 calendar months. But because months vary in length and doctors count from a date that’s actually before conception, the real answer has a few layers worth understanding.
Why Doctors Count 40 Weeks, Not 9 Months
The standard pregnancy timeline starts from the first day of your last menstrual period, not from the day you actually conceived. Since ovulation typically happens about two weeks into a 28-day cycle, you aren’t technically pregnant during the first two weeks of your “pregnancy.” The baby is really developing for about 38 weeks, but the 40-week convention gives doctors a consistent starting point for tracking growth and setting a due date.
This is why pregnancy feels like it stretches beyond 9 neat months. Forty weeks is closer to 9 months and 1 week on the calendar. Some sources describe pregnancy as lasting 10 lunar months (each 28 days), which also adds up to 280 days. Neither number is wrong; they’re just different ways of slicing the same timeline.
How Your Due Date Is Calculated
The most common method is a simple formula: take the first day of your last period, count back three calendar months, then add one year and seven days. So if your last period started on March 10, you’d count back to December 10, then add a year and seven days to get December 17. This assumes a regular 28-day cycle with ovulation on day 14, which doesn’t apply to everyone. If your cycles are longer or shorter, the estimate may shift by several days.
An early ultrasound, usually done in the first trimester, is the most reliable way to confirm or adjust a due date. It measures the embryo’s size and can pin down gestational age more accurately than cycle math alone, especially for people with irregular periods.
The Three Trimesters
Pregnancy divides into three trimesters of roughly three months each:
- First trimester (weeks 1 through 12): The brain and spinal cord begin forming. By the end of this stage, the arms, hands, fingers, feet, and toes are fully shaped. The embryo transitions to being called a fetus around week 10.
- Second trimester (weeks 13 through 27): External anatomy is fully developed and fingerprints start forming around month 4. By month 5, the fetus is covered in fine, downy hair. The lungs form their basic structure by month 6, though they aren’t mature enough to function outside the uterus.
- Third trimester (weeks 28 through 40): The fetus can open and blink its eyes around month 7 and begins regulating its own body temperature by month 8. In the final weeks, bones harden everywhere except the skull, which stays soft and flexible for delivery.
What “Full Term” Actually Means
Not every delivery at 9 months counts as full term in medical terms. Doctors use specific week ranges to classify when a baby arrives:
- 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
- Postterm: 42 weeks and beyond
A baby born at 37 weeks is sometimes described casually as “full term,” but medically that’s early term. Those last two weeks matter: babies born at 39 weeks or later generally have better outcomes with breathing, feeding, and temperature regulation than those born even a week or two earlier.
When Babies Actually Arrive
Very few babies arrive on their exact due date. An analysis of medical records found that first-time mothers deliver at an average of 275.9 days (about 39 weeks and 3 days), while mothers who have given birth before average 274.5 days, roughly a day and a half sooner. The difference is small but consistent.
Several factors nudge pregnancy length in one direction or the other. First-time mothers face a slightly higher risk of delivering early compared to those who have given birth before, which may seem contradictory, but it reflects a wider spread in outcomes rather than a simple shorter average. Each additional year of maternal age also raises the chance of preterm birth by a small amount.
Preterm Birth Categories
Any delivery before 37 weeks is considered preterm. The World Health Organization breaks this down further:
- Moderate to late preterm: 32 to 37 weeks
- Very preterm: 28 to less than 32 weeks
- Extremely preterm: less than 28 weeks
Most preterm births fall into the moderate-to-late category. Babies in this range often need some extra support after delivery but typically do well. The earlier a baby arrives, the more intensive the care required and the longer the hospital stay.
Month-by-Month Development at a Glance
Because 40 weeks spans parts of 10 calendar months, some pregnancy guides describe development across 10 months rather than 9. Here’s one milestone per month to give you a sense of the pace:
- Month 1: The amniotic sac forms, creating a cushioned, fluid-filled environment.
- Month 2: The brain and spinal cord take shape.
- Month 3: Fingers and toes separate and are fully formed.
- Month 4: Fingerprints begin developing.
- Month 5: Fine hair covers the body.
- Month 6: Lungs are structurally complete but not yet functional.
- Month 7: Eyes can open and blink.
- Month 8: The fetus starts controlling its own body temperature.
- Month 9: Bones harden, except for the skull plates.
- Month 10: Toenails reach the tips of the toes, and the baby is ready for delivery.
Growth accelerates dramatically in the third trimester. A fetus roughly doubles its weight between weeks 28 and 40, which is why those final weeks feel physically so much heavier for the person carrying the pregnancy.