A standard pregnancy lasts 40 weeks, or about 280 days, counted from the first day of your last menstrual period. That’s roughly nine calendar months, though the actual time your baby spends developing after conception is closer to 38 weeks. In practice, healthy pregnancies vary by several weeks in either direction, and most women don’t deliver on their exact due date.
Why Pregnancy Is Counted From Your Last Period
The 40-week figure starts from the first day of your last menstrual period (LMP), not from the day you conceived. This is called gestational age, and it’s the standard your doctor uses to track your pregnancy and estimate your due date. The reason is practical: most women know when their last period started, but pinpointing the exact day of conception is much harder.
Conception typically happens about two weeks after the start of your period, when you ovulate. That means your baby’s actual age, sometimes called fertilization age, is roughly two weeks less than the gestational age your doctor quotes. So when someone says they’re “12 weeks pregnant,” the embryo has been developing for about 10 weeks. This two-week gap is built into the system and doesn’t change anything about your care or due date.
The Three Trimesters
Pregnancy is divided into three trimesters, each covering roughly 13 to 14 weeks:
- First trimester: First day of your last period through 13 weeks and 6 days
- Second trimester: 14 weeks through 27 weeks and 6 days
- Third trimester: 28 weeks through 40 weeks and 6 days
Each trimester brings distinct stages of fetal development and different physical changes for you. The first trimester is when organs begin forming. By the second trimester, you can typically feel movement. The third trimester is a period of rapid growth and weight gain for the baby as it prepares for birth.
Most Women Don’t Deliver on Their Due Date
The 40-week mark is an estimate, not a deadline. Research tracking healthy women with known conception dates found that the median time from ovulation to birth was 268 days, or 38 weeks and 2 days. Measured from the last menstrual period, a 2013 study found the median was actually 285 days, which is 40 weeks and 5 days, not the even 40 weeks often quoted.
A large study of over two million pregnancies found that first-time mothers averaged 275.9 days of gestation, while women who had given birth before averaged 274.5 days. That’s only about a day and a half difference on average, but the gap is more noticeable at the extremes. Among first-time mothers, 6.2% delivered past 41 weeks. For women with a prior birth, only 4% went that long. In other words, first babies tend to take a bit more time.
Half of first-time mothers who go into labor on their own will deliver by 40 weeks and 5 days, and 75% will deliver by 41 weeks and 2 days. For women who’ve given birth before, those milestones come about two days earlier.
How Much Healthy Pregnancies Can Vary
Even among healthy pregnancies with no complications, the natural range is surprisingly wide. One study that tracked women with precisely known conception dates found that pregnancy length varied by as much as 37 days, even after excluding preterm births and pregnancies with complications. That’s more than five weeks of normal variation.
Several factors influence where you fall in that range. First pregnancies tend to run slightly longer, as noted above. Maternal age plays a role as well, with older mothers generally carrying a bit longer. Your individual biology, including the timing of early embryo development, also contributes. These aren’t things you can control, and a pregnancy that runs a week or two past the due date is not automatically a problem.
What “Full Term” Actually Means
Doctors don’t treat all weeks of pregnancy equally. The American College of Obstetricians and Gynecologists breaks the term period into specific categories:
- 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 distinctions matter because babies born at 39 weeks have better outcomes than those born at 37 weeks, even though both were once lumped together as “term.” The final weeks of pregnancy are important for brain development, lung maturity, and building fat stores that help regulate body temperature after birth. This is why elective deliveries are generally not recommended before 39 weeks without a medical reason.
How Due Dates Are Calculated
The most common method is Naegele’s rule: take the first day of your last menstrual period, add 280 days (or count forward 40 weeks), and that’s your estimated due date. This assumes a 28-day cycle with ovulation on day 14, which doesn’t apply to everyone. If your cycles are longer or shorter, your due date may be adjusted.
An early ultrasound, typically done in the first trimester, is considered more accurate than the LMP method. It measures the embryo’s size and compares it to standard growth charts. If the ultrasound date and the LMP date differ by more than about a week, your provider will usually go with the ultrasound estimate.
For pregnancies conceived through IVF, dating is especially precise because the exact day of embryo transfer is known. The calculation accounts for whether a day-3 or day-5 embryo was transferred, removing the guesswork that comes with natural conception.
Nine Months, Ten Months, or Something Else?
The casual shorthand of “nine months” is a bit misleading. Forty weeks is actually closer to nine months and one week when counted on a calendar. And since the count starts two weeks before conception, the baby is developing for roughly eight months and three weeks. Some people say pregnancy is “ten lunar months” because ten cycles of 28 days equals 280 days, which is technically correct but not how most people think about months. The simplest answer: expect about nine and a half calendar months from your last period to delivery, give or take a couple of weeks in either direction.