A full pregnancy lasts 40 weeks, or about 280 days, counted from the first day of your last menstrual period. That said, the medical definition of “full term” is narrower than most people realize: it covers only weeks 39 through 40 and 6 days. Babies born before or after that window fall into different categories, and the distinction matters for health outcomes.
Why the Count Starts Before Conception
The 40-week number surprises many people because it doesn’t start at conception. It starts on the first day of your last period, roughly two weeks before ovulation and fertilization actually happen. This method, based on a formula called Naegele’s Rule, assumes a 28-day menstrual cycle. If your cycles are longer or shorter, your actual due date may shift by several days in either direction.
In practice, only about 5% of babies arrive on their exact due date. Most births happen somewhere in the range of 37 to 42 weeks, and that entire window is considered normal. First-time mothers tend to go slightly longer, averaging about 276 days (just over 39 weeks and 3 days), compared to roughly 274.5 days for those who have given birth before.
The Four Categories of Term Pregnancy
In 2013, major medical organizations stopped using “term” as a single label and broke it into more 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
This matters because those final weeks of development make a real difference. Babies born at 37 weeks are twice as likely as those born at 38 weeks, and three times as likely as those born at 39 or 40 weeks, to die in the first month of life. The brain, lungs, and liver undergo significant maturation between weeks 37 and 39. That’s why doctors no longer treat 37 weeks as interchangeable with 40, and why elective deliveries are generally not scheduled before 39 weeks unless there’s a medical reason.
How Your Due Date Is Determined
Your provider will typically estimate your due date using two methods: the date of your last period and an ultrasound measurement. A first-trimester ultrasound, done at or before 13 weeks and 6 days, is the most accurate way to establish gestational age. In one study, 40% of women who received a first-trimester ultrasound had their due date adjusted because it differed from their period-based estimate by more than five days.
Later ultrasounds are less precise. A third-trimester scan generally won’t change your due date unless it suggests a discrepancy of more than 21 days. The earlier the measurement, the more reliable it is, because embryos grow at very consistent rates in the first trimester while variation increases as pregnancy progresses.
For IVF pregnancies, the calculation is simpler. Since the exact date of embryo transfer is known, there’s no need to estimate ovulation or count backward from a last period. The gestational clock starts from the transfer date (or egg retrieval date), adjusted for the age of the embryo at transfer.
What Happens After 41 Weeks
Going past your due date is common and not automatically a problem, but providers monitor more closely once you enter late-term territory at 41 weeks. Induction may be offered at 41 weeks, especially if the cervix shows signs of being ready for labor. If pregnancy continues past 42 weeks, induction is recommended and is generally performed no later than 42 weeks and 6 days.
Post-term pregnancies carry increased risks because the placenta gradually becomes less efficient at delivering oxygen and nutrients. The baby also continues to grow, which can complicate delivery. This is why the medical definition of a full pregnancy tops out well before 42 weeks, even though some pregnancies would naturally continue longer.
Weeks vs. Months: Why the Numbers Feel Off
One source of confusion is converting weeks to months. Forty weeks doesn’t neatly equal nine months. Since most calendar months are longer than four weeks, 40 weeks works out to about nine months and one week. Pregnancy apps and books sometimes describe pregnancy as lasting “nine months,” but the week count is what your provider tracks because it’s far more precise. When you hear someone say they’re “ten months pregnant,” they’re likely in the 41st week and rounding up, but they’re not wrong to feel that way.