Pregnancy week-by-week

How pregnancy weeks are counted

Pregnancy is dated from the first day of your last period, not from the day you conceived — which is worth knowing before you pick a week below, because it puts everything about two weeks ahead of where most people expect. At four weeks pregnant, conception was roughly two weeks ago, and the test you have just taken is reacting to a few days of hormone rather than a month of it.

The convention exists because nearly everyone knows when their last period started and almost nobody knows the hour they conceived. It has the odd consequence that week one is a week you were not pregnant at all: the counting starts before the thing it counts. Forty weeks from that first day is 280 days, and that is where a due date comes from.

If your cycles are long, short or irregular, that arithmetic drifts. An ultrasound in the first twelve or thirteen weeks measures the baby directly instead of trusting the calendar, and where an early scan disagrees with your dates by more than a few days, your provider will usually move the due date to match the scan. Later on, measurements vary enough between healthy babies that the original date is normally kept — which is why a third-trimester scan showing a big or small baby does not change how many weeks you are. ACOG sets out how dates are assigned and re-dated.

Trimester boundaries are a convention too, and sources genuinely differ by a week either way — so a page elsewhere calling week 13 the second trimester is not contradicting this one. The grouping below uses weeks 1–12, 13–26 and 27–40.

One more piece of vocabulary, because it catches people out at the end: full term is narrower than it sounds. Weeks 37 and 38 are early term, 39 and 40 are full term, week 41 is late term, and 42 onward is post-term. The distinction is why a provider may be relaxed about going past your date and still want to talk about induction a week later.

And a due date is an estimate, not an appointment. Most babies arrive in the fortnight either side of it and only a small fraction on the day itself, so treat the week you are reading as a rough position rather than a schedule.