A standard pregnancy lasts 40 weeks, which works out to about 9 calendar months, though it’s closer to 10 lunar months (months of exactly 4 weeks). That discrepancy is why you’ll see both numbers floating around, and neither is wrong. The 40-week count equals 280 days from the first day of your last menstrual period.
Why Pregnancy Seems Like Both 9 and 10 Months
The confusion comes from how months and weeks don’t divide neatly. If every month were exactly 4 weeks, 40 weeks would equal 10 months. But most calendar months are 30 or 31 days, making them slightly longer than 4 weeks. So 40 weeks spans roughly 9 calendar months and one extra week.
There’s another wrinkle: pregnancy is dated from the first day of your last menstrual period, not from conception. Conception typically happens about 2 weeks after that date, around ovulation. So while the clock starts ticking at 40 weeks, the baby actually spends closer to 38 weeks developing in the womb. Those first 2 weeks of “pregnancy” happen before you’re even pregnant.
How the Three Trimesters Break Down
Pregnancy is divided into three trimesters of roughly 3 months each. The first trimester covers weeks 1 through 12, the second runs from week 13 through week 27, and the third stretches from week 28 to delivery around week 40. These divisions aren’t arbitrary. Each trimester marks a distinct phase in how the baby grows and how your body changes.
What Happens Each Month
During the first month, the fertilized egg implants in the uterus and a fluid-filled sac forms around it for protection. By month 2, the brain, spinal cord, and central nervous system are taking shape. Month 3 brings the beginnings of teeth and taste buds.
The second trimester is when things get more noticeable. In month 4, vocal cords form and the baby’s head starts to grow more proportional to the body. By month 5, the brain regions responsible for the five senses begin developing. Month 6 is when bone marrow starts producing blood cells.
The third trimester is about maturing and getting ready for life outside the womb. In month 7, the lungs begin producing surfactant, a substance critical for breathing after birth. By month 8, the baby can regulate its own body temperature. In month 9, bones harden throughout the body, except for the skull plates, which stay soft and flexible so the baby can fit through the birth canal. If pregnancy extends to a 10th month (weeks 39 to 40), the final details fill in, down to toenails reaching the tips of the toes.
How Due Dates Are Calculated
The standard method for estimating a due date, called Naegele’s Rule, is straightforward. Take the first day of your last period, count back 3 calendar months, then add 1 year and 7 days. So if your last period started on March 10, you’d count back to December 10, then add a year and 7 days for a due date of December 17.
This formula assumes a regular 28-day menstrual cycle with ovulation on day 14. If your cycles are shorter or longer, the estimate may need adjusting. Ultrasound measurements early in pregnancy can also help refine the timeline, especially if your cycle length is irregular or you’re unsure about the date of your last period.
What “Full Term” Actually Means
Not all deliveries at or near 40 weeks are treated the same. The American College of Obstetricians and Gynecologists breaks “term” pregnancy into specific 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
- Post-term: 42 weeks and beyond
These categories exist because babies born even a few weeks early can face different health outcomes than those born at 39 or 40 weeks. The final weeks of pregnancy are when the brain, lungs, and liver go through important finishing stages of development. A baby born at 37 weeks is technically “term,” but the optimal window is 39 to 40 weeks.
Of course, 40 weeks is an average, not a deadline. Healthy pregnancies commonly run anywhere from 38 to 41 weeks. Only about 5% of babies arrive on their exact due date.