Pregnancy Weeks to Months: How the 40-Week Calendar Works

Pregnancy is measured in weeks, but everyone around you asks in months, and the two systems refuse to line up neatly. A pregnancy is 40 weeks, yet everyone calls it nine months — and 40 weeks divided by four is ten. The confusion is not your arithmetic. It comes from where the count starts, what a "month" means, and the difference between a four-week month and a calendar month. Here is how the 40-week calendar actually works. One note before we start: this is general information about how dates are counted, not medical advice — questions about your own pregnancy belong with your doctor or midwife.

The clock starts before conception

Pregnancy weeks are counted from the first day of the last menstrual period (LMP) — not from conception. That sounds odd, because for roughly the first two weeks of the count, there is no pregnancy yet. Conception typically happens around ovulation, about two weeks after the LMP in a textbook 28-day cycle.

The reason is practical: most people know the date their last period started, while the exact date of conception is usually unknowable. So medicine standardized on the date everyone can actually point to. The consequence is a built-in two-week offset:

"4 weeks pregnant" means roughly 2 weeks after conception — often right when a test first turns positive.
"12 weeks pregnant" means roughly 10 weeks of actual embryonic development.

Every number in this article uses that LMP-based count, because that is what clinicians, apps, and ultrasound reports generally use.

Why 40 weeks is not 10 months — or exactly 9

Forty weeks is 280 days. If you count in 28-day months — four weeks apiece, often called "lunar months" in older obstetric tradition, though an astronomical lunar month is actually closer to 29.5 days — 280 days is exactly 10 of them, which is where "ten months" traditions come from. But calendar months average about 30.4 days, so 280 days works out to roughly 9 calendar months plus about a week.

That is why "nine months" and "40 weeks" both circulate and both are approximately right. It is also why there is no clean weeks-to-months conversion chart: a calendar month is about 4.3 weeks, not 4. If you want a rough month figure, divide the week number by 4.3 — week 26 is about six months along, not six and a half.

Trimester boundaries

Pregnancy is conventionally split into three trimesters. A common division is:

First trimester: weeks 1–12
Second trimester: weeks 13–27
Third trimester: week 28 until delivery

Be aware that the exact cut points vary by source — some place the end of the first trimester at week 13 or 14, and the second-to-third boundary is sometimes drawn at week 27 or 28. The boundaries are conventions for organizing care and milestones, not biological switches, so do not be surprised if your clinic's dates differ slightly from an app's. If you are trying to see when a boundary lands on a real calendar, a page like 12 weeks from today converts the week count into an actual date instantly.

Preterm, early term, and full term

The weeks near the end are graded more finely, because the distinctions matter clinically:

Preterm: birth before 37 weeks
Early term: 37 through 38 weeks
Full term: 39 through 40 weeks
Late term: week 41, and postterm from 42 weeks

These categories were tightened by obstetric organizations precisely because 37 weeks and 40 weeks are not equivalent — "term" was redefined so that full term means 39–40 weeks, not simply "made it to 37."

The due date is a calculation, not a promise

The classic method is Naegele's rule: take the first day of the LMP and add 280 days. The traditional shortcut — add one year, subtract three months, add seven days — usually lands on the same date, though it can drift by a day or two because calendar months are not all the same length and leap years interfere (a classic example of the pitfalls covered in our guide to leap years and date math). The rule assumes a 28-day cycle with ovulation on day 14; if your cycles run longer or shorter, the estimate shifts, which is one reason a first-trimester ultrasound is often used to confirm or adjust the date.

Either way, the due date is the midpoint of a range, not an appointment. Only a small minority of babies arrive on the exact date; arriving a week or two on either side of it is entirely ordinary. Treat the due date as the center of a window rather than a deadline to plan around to the day.

If you want to run the numbers yourself, DateSum's calculators handle the tedious part. Enter the first day of the LMP into the days between dates calculator to see exactly how many days — and therefore weeks — have elapsed, or jump straight to 40 weeks from today to see where a full 280-day count lands on the calendar.