A typical pregnancy is counted as 40 weeks, measured from the first day of your last menstrual period rather than from the moment you actually conceived. That's the standard medical convention, and it's what every due date calculator, including ours, is built on. Forty weeks works out to about 280 days, or roughly 9 months and a week.
It feels like it should be a tidy 9 months, and in a loose, everyday sense it is. But "9 months" and "40 weeks" aren't quite the same length, and that small mismatch trips a lot of people up early on.
By the end of this article, you'll know exactly how the 40-week count works, how it maps onto months and trimesters, and why your due date is a well-informed estimate rather than a fixed appointment.
Key takeaways
- A full-term pregnancy is typically counted as 40 weeks, or about 280 days, measured from the first day of your last menstrual period.
- Forty weeks works out to roughly 9 months and one week, not an even 9 months, because calendar months vary in length while pregnancy weeks don't.
- Pregnancy weeks start counting from your last period, which is usually about two weeks before conception actually happens.
- Only around 4% of babies are born on their exact due date, so the 40-week mark is a estimate, not a deadline.
- Babies born from 39 weeks through 40 weeks are considered full term; before 37 weeks is preterm, and after 42 weeks is post-term.
How long is a pregnancy, really?
A pregnancy is typically considered to last 40 weeks, or 280 days, counted from the first day of your last menstrual period, not from the day you conceived. That means for roughly the first two weeks of your "pregnancy," you weren't actually pregnant yet — you hadn't ovulated.
This counting method, called gestational age, is the one your provider will use throughout. It's also what the due date calculator on this site uses, following the same standard formula described by ACOG's committee opinion on estimating due dates.
The reason it's done this way is practical. Most people know the date of their last period far more reliably than the exact day they conceived, so it gives everyone a consistent, comparable starting point.
How many months is 40 weeks pregnant?
Forty weeks of pregnancy works out to about 9 months and one week — not an even nine months — because calendar months run between 28 and 31 days while pregnancy weeks are always exactly 7 days long. If you divide 40 weeks by the average 4.3 weeks per month, you land just past the nine-month mark.
This is one of the most common sources of confusion in early pregnancy. "Due in month nine" and "40 weeks pregnant" describe the same finish line, but they don't divide evenly against each other.
Here's roughly how the weeks map onto months, using average 4.3-week months:
| Pregnancy weeks | Approximate months | Trimester |
|---|---|---|
| Weeks 1–4 | Month 1 | First |
| Weeks 5–8 | Month 2 | First |
| Weeks 9–13 | Month 3 | First |
| Weeks 14–17 | Month 4 | Second |
| Weeks 18–22 | Month 5 | Second |
| Weeks 23–27 | Month 6 | Second |
| Weeks 28–31 | Month 7 | Third |
| Weeks 32–35 | Month 8 | Third |
| Weeks 36–40 | Month 9 (plus a few days) | Third |
These ranges are approximate, since months don't divide neatly into 7-day weeks. Your provider will almost always talk to you in weeks, not months, because it's the more precise unit.
How are pregnancy weeks divided into trimesters?
Pregnancy is typically split into three trimesters: weeks 1 through 13 (first), weeks 14 through 27 (second), and weeks 28 through 40 (third). Some providers draw the lines a week or two differently, and that's normal — the trimester boundaries are a convenience, not a medical cutoff.
The first trimester covers the earliest and often most symptomatic stretch, as hormone levels climb steeply and the placenta establishes itself. If you want week-by-week detail on what's typical during this stretch, see our guide to first trimester symptoms week by week.
The second trimester is when many people start to feel more like themselves, and it's when the 20-week anatomy scan takes a detailed look at your baby's development. The third trimester, from week 28 on, is the run-up to birth, when appointments get more frequent and your provider watches more closely for things like blood pressure changes.
Why isn't your due date a guarantee?
Your due date is a statistical estimate of when labor is most likely, not a prediction of the exact day you'll deliver. Only around 4% of babies are born on their calculated due date, and a wide window on either side is considered completely normal.
Babies born from 39 weeks 0 days through 40 weeks 6 days are generally considered full term. Birth from 37 weeks through 38 weeks 6 days is called early term, before 37 weeks is preterm, and after 42 weeks is post-term — all categories used by ACOG and NHS guidance on due dates.
An ultrasound in the first trimester can refine the estimate if your cycle is irregular or you're unsure of your last period's start date. But even then, it's still an estimate — pregnancy length varies naturally from person to person, and that variation doesn't mean anything is wrong.
Frequently Asked Questions
Why is pregnancy 40 weeks and not 9 months?
Pregnancy is measured in exact 7-day weeks, while a calendar month is an uneven unit of 28 to 31 days. Forty weeks comes out to roughly 9 months and a week, so "40 weeks" and "9 months" describe nearly the same length of time, just counted with different rulers.
How many months is 40 weeks?
Forty weeks is approximately 9 months and one week, using an average month length of about 4.3 weeks. It's rarely an exact nine months, which is why providers almost always talk in weeks instead.
When do the weeks of pregnancy start counting?
Pregnancy weeks start counting from the first day of your last menstrual period, typically about two weeks before you actually ovulate and conceive. This is called gestational age, and it's the standard every due date calculator and provider uses, rather than counting from conception itself.
Is it normal for my due date to change?
Yes — it's common for your due date to shift slightly after an early ultrasound, especially if your cycles are irregular. A small adjustment doesn't mean anything is wrong; it just means the estimate has been refined with more direct information.
What counts as full term?
Full term is generally defined as birth between 39 weeks 0 days and 40 weeks 6 days. Babies born from 37 to 38 weeks 6 days are considered early term, and your midwife or doctor can explain what timing means for your specific pregnancy.
The Bottom Line
A typical pregnancy runs about 40 weeks, or 9 months and a bit, counted from your last period rather than conception. For a personalized estimate based on your own dates, try the due date calculator — and remember it's a well-grounded estimate, not a fixed appointment.