Pregnancy weeks to months converter

Why weeks will not divide neatly into months

Forty weeks is 280 days. A calendar month averages 30.44 days — that is 4.35 weeks, not four. Divide 280 by 30.44 and you get 9.2 months. Divide 40 weeks by four and you get ten. Both sums are arithmetically correct, and only one of them matches what anybody says out loud, which is why this conversion feels broken every time you attempt it.

The four-week version has a name: the lunar month. Ten lunar months is where the old “ten moons” framing comes from, and some obstetric textbooks still lay out fetal development that way. It is not what your mother means when she asks how many months along you are. She means calendar months, and by that measure a full-term pregnancy is a little over nine.

How the standard chart resolves it

Because 40 does not divide evenly into nine, every published chart builds the nine months from a mix of four-week and five-week blocks: 4 + 4 + 5 + 4 + 5 + 5 + 4 + 4 + 5 = 40. Four of the nine months are four weeks long and five of them run to five. That is the whole trick, and it is why the answer jumps in an uneven rhythm as you move through the pregnancy. This calculator uses the mapping published by The Bump, which Flo and most week-by-week trackers match.

Trimesters are the part that is genuinely standardised

Months are convention; trimesters are definition. The American College of Obstetricians and Gynecologists puts the first trimester from the first day of your last period through 13 weeks 6 days, the second from 14 weeks 0 days through 27 weeks 6 days, and the third from 28 weeks 0 days to birth. Those boundaries do not move between sources, so if you need one number that will not be argued with, give the trimester rather than the month.

ACOG also replaced the single word “term” with four categories, because outcomes differ across them: early term is 37w0d to 38w6d, full term 39w0d to 40w6d, late term 41w0d to 41w6d, and post-term 42w0d and beyond. The calculator reports which one you are in once you pass 37 weeks.

Where the charts disagree, and why nobody arbitrates

Published charts differ by up to a week at the month boundaries. The Mother Baby Center starts month 6 at week 24 rather than 23, month 8 at week 31 rather than 32, and month 9 at week 37 rather than 36. That is not an error on anyone’s part. There is no governing body for the weeks-to-months conversion because clinicians never make it: the NHS, ACOG and your own maternity notes count in completed weeks and days throughout. “20+3” appears on every page of your record; “five months” appears on none of them.

Practically, that means two things. If you are telling a midwife or a doctor how far along you are, say the weeks and days — “twenty and three” is unambiguous, whereas “five months” could mean anything from week 18 to week 23 depending on whose chart the other person is carrying. And if a friend or an app tells you a different month from the one below, both of you are probably right. The table flags every week where the sources part company.

The full mapping, weeks 1 to 42

What weeks 41 and 42 are

Nine months runs out at week 40, so weeks 41 and 42 sit off the end of every chart. Nothing calls them a tenth month; your notes simply keep counting. Most maternity units offer a membrane sweep around 40 to 41 weeks and discuss induction between 41 and 42 weeks, with extra monitoring if you prefer to wait. Only about four babies in a hundred arrive on their actual due date, and the great majority arrive somewhere in the two weeks either side of it.

A note on the first month

Weeks 1 and 2 are counted before you conceived. Dating runs from the first day of your last period because that is the date most people can actually remember, whereas ovulation usually is not observed. It means you are recorded as “four weeks pregnant” at roughly the moment a test first turns positive, and about two of those four weeks are bookkeeping. Every source uses the same convention, so the month numbers below already account for it.