Pregnancy lasts about 40 weeks, or 280 days, counted from the first day of your last menstrual period. That works out to roughly nine calendar months, though the actual time your baby spends developing is about two weeks shorter since conception typically happens around two weeks after your period starts. A normal pregnancy can range anywhere from 37 weeks to 42 weeks.
Why Pregnancy Is Counted From Your Last Period
The 40-week count begins on the first day of your last menstrual period (LMP), not the day you actually conceived. This sounds odd, but it exists because most people can remember when their period started while the exact moment of conception is nearly impossible to pin down. By the time a pregnancy test turns positive, you’re typically already considered four or five weeks pregnant under this system.
The actual time from conception to birth is closer to 38 weeks. Those first two weeks of the “40-week pregnancy” are the gap between your period starting and ovulation occurring. So when someone says they’re 12 weeks pregnant, the embryo has really been developing for about 10 weeks.
The Three Trimesters
Pregnancy is divided into three trimesters, each with a distinct phase of development:
- First trimester (weeks 4 through 12): Organs begin forming, the heart starts beating, and most of the early pregnancy symptoms like nausea and fatigue hit hardest. Miscarriage risk drops significantly after this period.
- Second trimester (weeks 13 through 27): Often called the most comfortable stretch. The baby grows rapidly, you’ll start to show, and most people begin feeling movement around weeks 18 to 22.
- Third trimester (weeks 28 through 41): The baby gains weight quickly, the lungs mature, and the body prepares for labor. This is when discomfort from the baby’s size tends to peak.
What “Full Term” Actually Means
Not all weeks at the end of pregnancy are equal. The American College of Obstetricians and Gynecologists breaks the final stretch into specific categories because a baby’s brain, lungs, and liver continue maturing right up to delivery:
- 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
This distinction matters because babies born at 39 weeks or later generally have fewer breathing problems, better temperature regulation, and stronger feeding ability than those born at 37 or 38 weeks. A pregnancy that reaches 37 weeks is considered safe, but the sweet spot is 39 to 40 weeks.
First Babies Often Take Longer
If this is your first pregnancy, you may go a bit past your due date. Data from a large analysis of birth 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. That’s roughly a day and a half difference, which is small but statistically consistent. First babies are simply more likely to arrive late rather than early.
How Accurate Is Your Due Date?
Your due date is an estimate, not a deadline. The traditional method adds 280 days to the first day of your last period, but this assumes a textbook 28-day cycle with ovulation on day 14. Many people don’t fit that pattern, and only about half of women accurately recall the exact start date of their last period.
A first-trimester ultrasound is the most accurate way to date a pregnancy, with a margin of error of plus or minus five to seven days. It’s accurate enough that in one study, 40% of women who received a first-trimester ultrasound had their due date adjusted by more than five days compared to what their period dates predicted. If you have an early ultrasound and it disagrees with your period-based date, your provider will likely go with the ultrasound.
Even with the best dating, only about 5% of babies arrive on their exact due date. Most are born within a week or two on either side of it.
What Happens If You Go Past 40 Weeks
Going past your due date is common and doesn’t automatically mean something is wrong. A pregnancy between 40 and 41 weeks doesn’t necessarily require extra monitoring, though your provider may want to check on the baby’s heart rate and fluid levels. Once you hit 41 weeks, testing is typically recommended, and labor induction is often discussed.
Induction is generally recommended if a pregnancy reaches 41 weeks because the placenta gradually becomes less efficient at delivering oxygen and nutrients past that point. Pregnancies that extend beyond 42 weeks are classified as post-term and carry higher risks, which is why very few providers will let a pregnancy continue past that mark without intervention.
Pregnancy Length in Months vs. Weeks
The “nine months” shorthand is a rough approximation. Forty weeks is actually closer to nine months and one week, since most calendar months are longer than four weeks. This is why you’ll hear people say pregnancy is “ten months” when counted in four-week blocks. Neither version is wrong; they’re just different ways of rounding the same 280 days. Doctors and midwives use weeks because they’re more precise, and a single week can matter when tracking development or timing delivery.