A typical pregnancy lasts about 280 days, or 40 weeks, starting from the first day of your last menstrual period (LMP).
Most people hear pregnancy lasts nine months. So it might come as a surprise to learn that a full-term pregnancy is actually 40 weeks — which works out to a bit more than nine calendar months. The difference matters when you’re trying to predict your due date or understand what “full term” really means.
The honest answer is about 280 days, but that number isn’t a hard deadline. A healthy pregnancy can range from 37 to 42 weeks, or 259 to 294 days. This article explains how pregnancy days are counted, why the number isn’t always exact, and what ranges doctors watch for.
How Pregnancy Length Is Measured
Providers count pregnancy from the first day of your last menstrual period (LMP), not from conception. That’s because the LMP date is usually the most reliable memory point for someone with a regular cycle. Adding 280 days (40 weeks) gives an estimated due date. This method assumes a 28-day cycle with ovulation around day 14.
Gestational age is the term for this measurement. It starts at day 1 of the LMP and runs through delivery. The baby’s actual age — called fetal age — is about two weeks shorter, roughly 266 days or 38 weeks from conception. ACOG explains that these extra two weeks are counted before ovulation occurs, which is why the 280-day number exists.
Why 40 Weeks Feels Longer Than 9 Months
If you’ve ever glanced at a calendar and thought “40 weeks isn’t exactly nine months,” you’re not wrong. A 280-day pregnancy starting January 1 would end around October 8 — that’s about nine months and one week. The confusion comes from medical counting versus calendar counting. Here are a few reasons the numbers don’t line up neatly:
- Calendar months vary in length: Four weeks is 28 days, but most months are 30 or 31 days. Forty weeks spreads across roughly 9.2 calendar months, so “due in nine months” is a loose estimate.
- The two-week pre-conception buffer: Day 1 of your LMP is about two weeks before conception. Those 14 days count toward gestational age but not fetal age, making the total seem longer than the actual time the baby spends growing.
- Normal variation is wide: Only about 4% of babies arrive on their exact due date. Most are born between 37 and 42 weeks. So a pregnancy could be 259 days or 294 days and still be considered healthy.
- Trimesters divide weeks differently: The first trimester covers weeks 1–12, the second weeks 13–27, and the third weeks 28–40. These chunks don’t correspond perfectly to three equal calendar months, which adds to the confusion.
Knowing why the math feels off can help you set realistic expectations for your due date. It’s common to go past 40 weeks, and your provider will monitor you closely if you do.
What the Due Date Really Tells You
An estimated due date (EDD) is a target, not a deadline. Providers use it to schedule tests, monitor growth, and decide when to intervene. The main categories of pregnancy length are defined by weeks from LMP, and each has medical significance. A baby born before 37 weeks is considered preterm, and the preterm baby definition from New York HEALTH notes that these babies require extra care. Conversely, a pregnancy beyond 42 weeks may be called post-term and needs close monitoring.
Here’s a quick reference table of the standard pregnancy length categories:
| Category | Weeks (from LMP) | Days (approximate) |
|---|---|---|
| Preterm | Less than 37 weeks | Less than 259 days |
| Early term | 37 weeks 0 days – 38 weeks 6 days | 259 – 272 days |
| Full term | 39 weeks 0 days – 40 weeks 6 days | 273 – 286 days |
| Late term | 41 weeks 0 days – 41 weeks 6 days | 287 – 293 days |
| Post-term | 42 weeks and beyond | 294 days and beyond |
Your provider will adjust due dates based on early ultrasound measurements, which are more accurate than LMP alone for predicting delivery timing. A due date is a helpful guide, but the real goal is a healthy pregnancy ending between 37 and 42 weeks.
Factors That Can Shift Your Due Date
Several factors can make your due date move up or down, either because the LMP method isn’t perfect or because your body follows a different rhythm. If your cycle is irregular or you ovulated later than day 14, the 280-day estimate might be off. Here are common factors that providers consider:
- Irregular menstrual cycles: If your cycle is longer than 28 days, you may ovulate later, and your due date may need to be adjusted forward. A 35-day cycle, for example, adds about a week to the typical calculation.
- Ovulation timing uncertainty: Even with a regular cycle, ovulation can vary by a few days from month to month. Ovulation predictor kits and tracking can help, but providers rely on early ultrasound for the most accurate dating.
- Early ultrasound correction: A first-trimester ultrasound measures the crown-rump length of the embryo, which is very consistent across pregnancies in early weeks. If the ultrasound date differs from the LMP date by more than a week, the provider will usually use the ultrasound date.
- Multiple pregnancy: Twins or triplets often arrive earlier than singletons, so the due date may be a week or two earlier than 40 weeks for multiples.
If you’re unsure about your dates, mention it at your first prenatal visit so your provider can use the best method to set your due date.
The Difference Between Gestational Age and Fetal Age
It’s easy to mix up gestational age and fetal age, but the distinction is simple once you see it. Gestational age counts from the first day of your LMP — it includes the two weeks before conception. Fetal age counts from the moment of conception. If you know your exact ovulation date, you can subtract about two weeks from the gestational age to get the fetal age. The table below shows how they compare across a typical pregnancy.
| Marker | Gestational Age (weeks from LMP) | Fetal Age (weeks from conception) |
|---|---|---|
| Start of pregnancy | Week 1 | – (pre-ovulation) |
| Conception occurs | Week 2–3 | Week 0 |
| First trimester ends | Week 12 | Week 10 |
| Full term begins | Week 39 | Week 37 |
| Typical due date | Week 40 | Week 38 |
While the term pregnancy range from the NHS defines a healthy window of 37 to 42 weeks (gestational age), the baby’s actual development aligns more closely with fetal age. That’s why a baby born at 37 weeks is considered early term but usually healthy — they’ve had about 35 weeks of fetal growth, which is enough for most organ systems to mature.
The Bottom Line
A typical pregnancy lasts about 280 days (40 weeks) from your last menstrual period. But the normal range is broader — 37 to 42 weeks (259 to 294 days). Your due date is an estimate, not a promise, and only about 4% of births happen on the exact day. The important thing is that your provider monitors your pregnancy within that healthy window and adjusts the plan if needed.
Your obstetrician or midwife can use your cycle history and early ultrasound to narrow down your personal due date. If you go past 40 weeks, they’ll keep a close eye on your baby’s wellbeing — but for most people, a few extra days are well within the normal range.
References & Sources
- New York HEALTH. “Why Is 40 Weeks So Important” A preterm or premature baby is delivered before 37 weeks of pregnancy (less than 259 days).
- NHS. “Due Date Calculator” A pregnancy that lasts between 37 weeks and 42 weeks (259 to 294 days) is considered a term pregnancy.