A standard pregnancy lasts 40 weeks, or 280 days, counted from the first day of your last menstrual period. That number surprises many people because conception itself typically happens around week 2, meaning the actual time a fetus develops is closer to 38 weeks. The 40-week convention exists because the start of your last period is a reliable, easy-to-identify date, while the exact moment of conception usually isn’t.
How the 40 Weeks Are Counted
Pregnancy dating starts from the first day of your last menstrual period (LMP), not from the day you conceived. This method assumes a 28-day menstrual cycle with ovulation on day 14. Since the first two weeks of “pregnancy” actually precede conception, you’re already considered two weeks pregnant by the time an egg is fertilized.
The standard formula for calculating a due date is called Naegele’s Rule. It works like this: take the first day of your last period, count back three calendar months, then add one year and seven days. If your last period started June 1, you’d count back to March 1, then add a year and seven days for a due date of March 8 the following year.
This formula has obvious limitations. It assumes a regular 28-day cycle, accurate recall of your last period, and ovulation on day 14. If your cycles are longer, shorter, or irregular, the estimate can be off by a week or more. That’s why ultrasound measurements in the first trimester are often used to adjust the due date, since early fetal size is a more precise indicator of gestational age.
The Three Trimesters
The 40 weeks break into three trimesters, each with distinct developmental milestones:
The first trimester spans weeks 1 through 12. This is the period of the most rapid and foundational development. By week 5, a tiny cluster of cells that will become the heart is already pulsing about 110 times a minute. By week 6, those pulses can be detected on an ultrasound. By the end of this trimester, all major organs and body systems have begun to form.
The second trimester covers weeks 13 through 27. This is when the fetus grows significantly in size and the pregnancy becomes visible. A critical threshold arrives around week 23: if born prematurely at this point, a baby may survive with intensive care. The lungs begin developing but aren’t functional yet. By week 26, the lungs start producing surfactant, a substance essential for breathing air after birth.
The third trimester runs from week 28 through delivery. During these final weeks, the lungs finish maturing, the brain develops rapidly, and the fetus gains the body fat needed to regulate temperature outside the womb. By month 9, the lungs are close to fully developed.
What “Full Term” Actually Means
For years, any birth between 37 and 42 weeks was lumped together as “term.” Medical organizations have since refined that into more specific categories, because outcomes differ meaningfully depending on exactly 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
- Post-term: 42 weeks and beyond
The distinction matters because babies born at 37 or 38 weeks, while generally healthy, have slightly higher rates of breathing difficulties and feeding problems compared to those born at 39 or 40 weeks. That two-week difference gives the brain and lungs meaningful extra development time.
Very Few Babies Arrive on Their Due Date
Only about 5% of babies are born on the exact due date. The 40-week mark is a midpoint of a range, not a precise prediction. Most deliveries happen sometime between 37 and 42 weeks, and that entire window is considered normal.
If your pregnancy reaches 41 weeks without signs of labor, your provider will typically discuss induction. Going past 42 weeks carries increased risks for both the baby and the mother, so most pregnancies are not allowed to continue beyond that point.
Why Some Pregnancies Are Naturally Longer or Shorter
Even when you account for cycle length and accurate dating, pregnancies vary by up to five weeks between individuals. Research published in Human Reproduction identified several biological factors that influence this natural variation. Older mothers tended to carry slightly longer. Women who had longer pregnancies in previous births were likely to have longer ones again, suggesting a personal biological pattern. Interestingly, women who were themselves heavier at birth also had longer gestations.
One surprising finding: how quickly the embryo implanted in the uterine wall predicted how long the rest of the pregnancy would last. Embryos that took longer to implant also took longer to reach delivery. The body’s hormonal response to implantation mattered too. Pregnancies where progesterone rose quickly after implantation lasted about 12 days longer on average than those with a slower hormonal response.
Factors you might expect to matter, like BMI, alcohol intake, whether it was a first or subsequent pregnancy, or the baby’s sex, did not show a significant association with pregnancy length in that study.
Counting Weeks vs. Counting Months
One common source of confusion is converting weeks to months. Forty weeks doesn’t divide neatly into nine calendar months because most months have more than four weeks. Forty weeks is actually closer to nine months and one week. This is why people sometimes say pregnancy lasts “nine months” while medical providers consistently use weeks. Weeks are more precise, which is why every prenatal appointment, ultrasound, and milestone is tracked in weeks rather than months.
If you’re trying to figure out what month you’re in, a rough guide: multiply the number of completed months by 4.3 to get the approximate week. At five months, you’re around week 21 or 22. At seven months, you’re near week 30. But if precision matters, especially for tracking fetal development or scheduling tests, stick with weeks.