Pregnancy Weeks to Months

A pregnancy is 40 weeks, but 40 ÷ 4 = 10, not 9. The catch is that a month averages about 4.35 weeks, not exactly 4. The prenatal convention maps 40 weeks onto 9 months by grouping the weeks unevenly — which is why "how many months is 30 weeks" does not have a clean answer, and month boundaries land mid-week.

The usual grouping runs: month 1 is weeks 1–4, month 2 is 5–8, month 3 is 9–13, then roughly 4–5 weeks per month through month 9 (weeks 36–40). The table above lays it out. This is the convention prenatal care uses, so it matches what a provider means by "you're in your fifth month".

Months group into three trimesters: the first covers months 1–3 (through 13 weeks), the second months 4–6 (14–27 weeks), and the third months 7–9 (28 weeks to birth). The calculator shows both the month and the trimester for the week you enter.

The week number you enter is doing all the real work here — it typically comes from an LMP-based estimate or an ultrasound, and carries whatever precision (or imprecision) that source has. Converting it to a month does not add accuracy; it only restates the same week count in a rounder, less precise unit. Two people both "in month 6" can be a couple of weeks apart in actual gestational age. If the week count itself changes later — say, after a dating ultrasound revises it — the month and trimester shown here shift with it automatically, since both are just read off the same number.

Because month boundaries are uneven and land mid-week, "five months pregnant" can mean anywhere from about week 18 to week 22 depending on where the count falls — too wide a range for anything clinical. That is why prenatal appointments, growth checks and test timing are all scheduled by week, not month. Ultrasound measurements, lab work and growth expectations are all referenced to a specific week — sometimes a specific day — which a month figure is simply too coarse to capture. Use the month for everyday conversation; for anything about your actual care or timeline, the week-based number from your provider is the one that matters.