A full-term pregnancy lasts 40 weeks, which works out to about 9 calendar months and one week, or 280 days. The count starts from the first day of your last menstrual period, not from the day you actually conceived, which means the first two weeks of “pregnancy” happen before conception even occurs.
Why It’s Counted in Weeks, Not Months
Doctors track pregnancy in weeks because months aren’t all the same length. February has 28 days, July has 31, and that inconsistency makes “nine months” a rough approximation at best. Weeks give a more precise timeline, which matters when tracking fetal development and planning for delivery. When someone says they’re “six months pregnant,” that could mean anywhere from 22 to 27 weeks depending on how they’re counting. Saying “24 weeks” removes the ambiguity.
That said, pregnancy is commonly divided into three trimesters of roughly three months each:
- First trimester: conception through 12 weeks
- Second trimester: 13 through 27 weeks
- Third trimester: 28 through 40 weeks
Gestational Age vs. Fetal Age
The 40-week count uses what’s called gestational age, which begins on the first day of your last menstrual period. Since ovulation typically happens around day 14 of a 28-day cycle, your baby’s actual age from conception is about two weeks shorter than the gestational age. So while the pregnancy is dated at 40 weeks, the baby has been developing for closer to 38 weeks.
This convention exists because most people can pinpoint when their last period started but not exactly when they ovulated or conceived. It’s the standard used across medicine, which is why ultrasound reports and prenatal visits all reference gestational age.
How Your Due Date Is Calculated
The most common method for estimating a due date follows a simple three-step formula. Take the first day of your last menstrual 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 land on December 17.
This formula assumes a regular 28-day menstrual cycle with ovulation on day 14. If your cycles are longer or shorter than 28 days, the estimate shifts accordingly. An early ultrasound, usually done in the first trimester, can refine the due date by measuring the embryo’s size. When there’s a discrepancy between the ultrasound and the period-based estimate, doctors often go with the ultrasound measurement.
What “Full Term” Actually Means
Not all deliveries at or near 40 weeks are considered the same. The American College of Obstetricians and Gynecologists breaks it down 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 distinctions matter because babies born even a couple of weeks early can face more health challenges than those born in the full-term window. A baby born at 37 weeks is technically “term,” but their lungs, brain, and liver are still maturing in meaningful ways during those final two to three weeks.
How Long Pregnancy Actually Lasts in Practice
Very few babies arrive on their exact due date. An analysis of delivery data found that first-time mothers carry an average of 275.9 days (about 39 weeks and 3 days), while mothers who have given birth before deliver slightly earlier, at an average of 274.5 days. That’s a small but statistically real difference, roughly a day and a half shorter for second or later pregnancies.
Normal, healthy pregnancies can vary by several weeks in either direction. Some babies arrive at 37 weeks with no complications, while others stay put until 41 weeks. The 40-week figure is a midpoint, not a deadline.
Month-by-Month Size Milestones
If you prefer thinking in months, here’s a sense of how the baby grows across the pregnancy:
By the end of month 2 (week 8), the embryo is about half an inch long, roughly the size of a kidney bean. All major organs have started forming. By month 3 (week 12), the fetus measures about 2 inches and weighs half an ounce. Fingers and toes are fully formed, and the risk of miscarriage drops significantly after this point.
By month 4 (week 16), the fetus is over 4 inches long and weighs more than 3 ounces. Many people start to feel the pregnancy become visible around this time. At month 5 (week 20), the halfway mark, the fetus is over 6 inches long. This is typically when you can find out the sex on ultrasound and start to feel movement.
During month 6, movements become stronger and more noticeable. The fetus can suck its thumb if a hand drifts near its mouth. The final trimester, months 7 through 9, is primarily about weight gain and organ maturation. The baby’s lungs are among the last organs to fully develop, which is one reason those final weeks of pregnancy make such a difference for newborn health.