A typical pregnancy lasts 40 weeks, or 280 days, counted from the first day of your last menstrual period. That’s roughly 9 calendar months, though the actual time from conception to birth is closer to 38 weeks. In practice, healthy pregnancies vary quite a bit. Only about half of women give birth within a few days of the 40-week mark, and the natural range can span more than five weeks.
Why Pregnancy Is Counted From Your Last Period
The 40-week clock starts on the first day of your last menstrual period, not the day you conceived. This is called gestational age, and it’s the standard doctors use worldwide. The reason is simple: most people know when their last period started, but very few know the exact day conception happened.
Since ovulation and conception typically occur about two weeks into a menstrual cycle, the actual biological pregnancy (sometimes called conceptional age) is roughly 266 days, or 38 weeks. So when your doctor says you’re “8 weeks pregnant,” the embryo has really been developing for about 6 weeks. This two-week gap catches many people off guard, but it’s baked into every due date calculation and trimester timeline you’ll encounter.
The Three Trimesters
Pregnancy is divided into three trimesters, each with distinct milestones:
- First trimester (weeks 4 through 12): Organs begin forming, and early pregnancy symptoms like nausea and fatigue are at their peak. This is also when miscarriage risk is highest.
- Second trimester (weeks 13 through 27): Often called the most comfortable stretch. The fetus grows rapidly, and most people start feeling movement around weeks 18 to 22.
- Third trimester (weeks 28 through 40+): The final 12 or so weeks, when the fetus gains the most weight and the body prepares for labor.
How Your Due Date Is Calculated
The most common method is Naegele’s rule: take the first day of your last period, add 280 days (or count forward 40 weeks), and that’s your estimated due date. It assumes a 28-day cycle with ovulation on day 14, which doesn’t fit everyone. About half of women don’t recall their last period date accurately, and many have cycles shorter or longer than 28 days.
That’s why a first-trimester ultrasound is considered the most accurate way to date a pregnancy. Measuring the embryo between 7 and 13 weeks gives an estimate accurate to within 5 to 7 days. In one study, 40% of women who had a first-trimester ultrasound had their due date adjusted by more than 5 days compared to the date calculated from their last period. If you only get an ultrasound in the second trimester, the margin of error widens.
When Babies Actually Arrive
Your due date is a midpoint estimate, not a deadline. A study tracking healthy pregnancies found that 50% of first-time mothers gave birth by 40 weeks and 5 days, and 75% by 41 weeks and 2 days. Women who had given birth before tended to deliver a couple of days earlier, with half giving birth by 40 weeks and 3 days.
A separate study that pinpointed ovulation timing found that even among healthy, uncomplicated pregnancies, the length from ovulation to birth ranged from 247 to 284 days, a span of 37 days. The standard deviation was about 9 days, meaning a “normal” pregnancy can easily fall a week or more on either side of the due date. Roughly 68% of women deliver within 11 days of their estimated date.
Factors like age, body mass index, and even the length of a previous pregnancy influence when labor starts naturally. If your first baby came late, your second is more likely to arrive closer to 40 weeks. Women with a short gap between pregnancies (under a year) tend to have their next pregnancy run about one day shorter for every three months the gap is under 12.
Early Term, Full Term, and Beyond
Not all “term” births are the same. The American College of Obstetricians and Gynecologists breaks it down like this:
- Early term: 37 weeks 0 days through 38 weeks 6 days
- Full term: 39 weeks 0 days through 40 weeks 6 days
- Late term: 41 weeks 0 days through 41 weeks 6 days
- Post-term: 42 weeks 0 days and beyond
These distinctions matter because babies born in the full-term window (39 to 40 weeks) generally have the best outcomes. Babies born at 37 or 38 weeks are often healthy but have slightly higher rates of breathing difficulties and feeding challenges compared to those born even one or two weeks later. This is why elective deliveries are typically not recommended before 39 weeks unless there’s a medical reason.
What Happens if Pregnancy Goes Past 42 Weeks
Only a small percentage of pregnancies extend past 42 weeks, but the risks do increase. The amniotic fluid can decrease, which may compress the umbilical cord and reduce oxygen flow to the baby. The baby may also grow larger than expected, increasing the chance of a difficult delivery or cesarean birth. In rare cases, the baby can inhale meconium (its first stool) into the lungs, which causes serious breathing problems after birth.
For the mother, post-term pregnancy raises the likelihood of infection, heavier postpartum bleeding, and the need for an assisted delivery. Stillbirth risk, while still low overall, also rises after 42 weeks. Because of these risks, most providers will discuss induction of labor if your pregnancy reaches 41 weeks, and will typically recommend it by 42 weeks.
Months vs. Weeks: Why the Math Feels Off
One of the most common points of confusion is how 40 weeks equals “9 months” when 40 weeks is actually closer to 10 calendar months. The disconnect comes from the fact that most calendar months are slightly longer than 4 weeks (averaging about 4.3 weeks). So 40 weeks works out to roughly 9 months and 1 week from your last period, or about 8 months and 3 weeks from conception. Doctors use weeks rather than months because weeks are more precise and avoid this kind of rounding confusion.