How many weeks is a pregnancy? Weeks, months and trimesters explained

Nine months, 40 weeks, ten months? The numbers don’t seem to add up. Here’s why, and how the weeks and trimesters work.

How long is a pregnancy?

Tommy’s says a pregnancy generally lasts around 40 weeks. The NHS says it normally lasts from 37 to 42 weeks from the first day of your last period. Babies arrive across that whole range, so your due date is a guide, not a deadline. See past your due date.

Why is pregnancy counted from my last period?

Tommy’s explains that pregnancy is measured from the first day of your last period rather than the day you conceived. It also says weeks are more accurate than talking in months. In practice you’re counted as ‘4 weeks pregnant’ around the time your period is due, even though conception happened about two weeks earlier. See working out your conception date.

The NHS due date calculator asks for the first day of your last period, and notes that your cycle may not be exactly 28 days, so you can adjust it. Your midwife will offer a scan, usually around 12 weeks, which the NHS says can estimate how many weeks pregnant you are more accurately. See 12-week scan.

Weeks and months

Forty weeks is a little over 9 months. Tommy’s says people often call pregnancy nine months, but it is usually slightly longer. That is why ‘how many months pregnant am I?’ is harder to answer than it sounds. If you want a rough guide, 4 weeks is about a month, so 20 weeks is a bit under 5 months.

Trimesters

A trimester is about three months. Tommy’s describes them as:

First trimester: weeks 1 to 12
Second trimester: weeks 13 to 27
Third trimester: weeks 28 to 40

The NHS week-by-week guide runs its first trimester pages from week 4 to 12, its second from 13 to 27, and its third from 28 to 41, since it includes a page for week 41. Different sources draw the lines slightly differently, which is why you may see small differences. Dearbump’s guides start at week 4. See your first few weeks, step by step.

Questions people ask

Is pregnancy 9 or 10 months?

Forty weeks is a little over 9 months. Tommy’s says it is usually slightly longer than nine months.

When do I count as 4 weeks pregnant?

Pregnancy is counted from the first day of your last period, so you are usually about 4 weeks pregnant when your period is due.

What if my cycle isn’t 28 days?

The NHS says your cycle may not be exactly 28 days and lets you change the cycle length in its calculator. Your 12-week scan can also give a more accurate dating.

How many weeks is the third trimester?

Tommy’s gives weeks 28 to 40. The NHS guide runs it to week 41.