Pregnancy Weeks to Months

Pregnancy weeks to months chart

Here is the standard mapping the calculator uses. Each pregnancy month is a block of four to five weeks, and each block sits inside one of the three trimesters. Use it as a quick lookup whenever someone asks what month you’re in.

Month 9 runs from week 36 to your due date around week 40, and a little beyond if baby takes their time — many pregnancies reach 41 or even 42 weeks.

How weeks, months, and trimesters fit together

Pregnancy is counted in weeks from the first day of your last menstrual period, not from conception. That convention keeps everyone — you, your midwife, every app and chart — on the same page. The catch is that a calendar month averages about 4.3 weeks, not a clean four, so the weeks and months never line up perfectly. Grouping weeks into four-and-five-week blocks is the standard fix, and it’s what you’ll see on almost every pregnancy site.

The three trimesters are the bigger milestones:

• First trimester — weeks 1 to 13 (months 1–3). The foundation stage, when major organs form.
• Second trimester — weeks 14 to 27 (months 4–6). Often the most comfortable stretch, with a growing bump and first kicks.
• Third trimester — weeks 28 to 40 (months 7–9). The home stretch, with rapid growth and getting ready for birth.

So if you’re third month pregnant you’re somewhere in weeks 9–13 and still in your first trimester, while month 7 (weeks 28–31) marks the very start of your third. The calculator and chart above turn any week into that month-and-trimester answer in one step.

You may notice that other charts vary by a week here or there — some start month 1 at conception rather than your last period, and some round the final stretch differently. That’s normal. There is no single official rule that splits 40 weeks into 9 tidy months, so each source picks a grouping and sticks with it. The four-and-five-week blocks on this page are the most widely used version and line up cleanly with the three trimesters, which makes them easy to remember and easy to compare against your app or your provider’s notes. If your numbers differ by a few days, it almost always comes down to which starting point and rounding convention the other chart chose, not a mistake on either side.

Common weeks people look up

• 8 weeks — month 2, first trimester.
• 12 weeks — month 3, first trimester (the classic end-of-trimester-one milestone).
• 20 weeks — month 5, second trimester (around the halfway point and the anatomy scan).
• 28 weeks — month 7, the first week of the third trimester.
• 36 weeks — month 9, considered full term is just ahead at 37–40 weeks.

Remember these are dating conventions, not a countdown set in stone. Your provider may adjust your dates after an early ultrasound, which can shift your week (and therefore your month) by a few days.