A standard pregnancy lasts 40 weeks, counted from the first day of your last menstrual period. That’s 280 days total, split across three trimesters. The number surprises many people because pregnancy is commonly described as lasting nine months, but the math works out to closer to ten calendar months when you count week by week.
Why 40 Weeks Instead of 9 Months
The confusion between weeks and months comes down to how calendar months work. Most months are a little longer than four weeks (30 or 31 days versus 28), so they don’t divide evenly into the 40-week timeline. In pregnancy terms, each “month” is treated as exactly four weeks. That gives you this breakdown:
- 4 weeks: 1 month
- 8 weeks: 2 months
- 12 weeks: 3 months
- 16 weeks: 4 months
- 20 weeks: 5 months
- 24 weeks: 6 months
- 28 weeks: 7 months
- 32 weeks: 8 months
- 36 weeks: 9 months
- 40 weeks: 10 months
So pregnancy is technically ten four-week months, not nine. The “nine months” shorthand is roughly accurate when you use calendar months, but healthcare providers track pregnancy in weeks because it’s far more precise.
How the Three Trimesters Break Down
The 40 weeks are divided into three trimesters, each marking a distinct phase of development.
The first trimester runs from the first day of your last period through 13 weeks and 6 days. This is when the major organs and structures begin forming. It’s also when most people experience nausea, fatigue, and breast tenderness.
The second trimester spans 14 weeks 0 days through 27 weeks and 6 days. Fetal movement becomes noticeable, energy levels often improve, and most anatomy scans happen during this window.
The third trimester covers 28 weeks 0 days through 40 weeks and 6 days. The baby gains most of its weight during this stretch, and you’ll feel increasing pressure on your bladder, lungs, and lower back as it grows.
How Your Due Date Is Calculated
Your estimated due date is set at 280 days (40 weeks) after the first day of your last menstrual period. This method assumes a 28-day cycle with ovulation happening on day 14, which means the count actually starts about two weeks before conception. Your baby’s true age is roughly two weeks younger than the “gestational age” used to track your pregnancy.
That built-in assumption doesn’t work perfectly for everyone. If your cycles are irregular, or you don’t remember the exact date of your last period, the initial estimate can be off. That’s where early ultrasound comes in. In one study, 40% of women who received a first-trimester ultrasound had their due date adjusted because it differed from the period-based estimate by more than five days. If there’s a significant gap between the two methods, the ultrasound measurement typically takes priority.
Not Every Pregnancy Hits Exactly 40 Weeks
Forty weeks is a target, not a timer. Most babies arrive within a window around that date, and the exact length varies from one pregnancy to the next. First-time mothers tend to go slightly longer, with an average pregnancy lasting about 275.9 days (just over 39 weeks and 3 days). Women who have delivered before average 274.5 days, about a day and a half shorter.
To reflect this natural variation, pregnancies that reach the general neighborhood of 40 weeks are classified more precisely:
- 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 matter because outcomes differ across them. Babies born at 39 or 40 weeks generally do better than those born at 37 or 38 weeks, even though all fall under what used to be lumped together as simply “term.” The distinction is why providers prefer not to schedule elective deliveries before 39 weeks unless there’s a medical reason.
What Happens Before 37 Weeks
A baby born before 37 weeks is considered preterm. The earlier the delivery, the greater the health risks. The World Health Organization breaks preterm birth into three categories:
- Extremely preterm: less than 28 weeks
- Very preterm: 28 to less than 32 weeks
- Moderate to late preterm: 32 to 37 weeks
Babies born in the moderate to late preterm range often do well with some extra support, while those born extremely preterm face much more significant challenges with breathing, temperature regulation, and feeding. Each additional week in the womb during the third trimester makes a measurable difference in lung maturity and brain development.
What Happens After 41 Weeks
Going past your due date is common and usually not dangerous on its own. But once a pregnancy reaches 41 weeks, providers typically recommend induction. A pregnancy that extends beyond 42 weeks is classified as post-term, and the risks begin to climb. These include decreased amniotic fluid (which can compress the umbilical cord and restrict oxygen), the baby passing its first stool before birth (which can cause breathing problems if inhaled), and a higher likelihood of the baby growing unusually large, making delivery more difficult.
The overall chance of serious complications in post-term pregnancies is still small, but monitoring becomes more frequent after 41 weeks. You can expect more frequent checks on the baby’s heart rate and amniotic fluid levels if you pass your due date. Most providers will have a conversation about induction well before you reach the 42-week mark.