Why is pregnancy 40 weeks but nine months?

Because a month averages about 4.3 weeks, so 40 weeks is roughly 9.2 months.

When does each trimester start?

The first is weeks 1–13, the second 14–27 and the third 28–40.

Pregnancy: Weeks to Months

Published 8/21/2025 · 2 min read · Health calculators

Sofia Nunes — Health & wellness writer at Allin

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In short

Pregnancy is counted in weeks — 40 in total — but converting to months is a bit messy because months aren't exactly four weeks. A rough guide is to divide the weeks by 4.3 for months. The three trimesters split it as weeks 1–13, 14–27 and 28–40. So 'how many months is 30 weeks?' is about 7 months, early in the third trimester.

Hands holding an ultrasound image over grass. — RDNE Stock project · Pexels · Pexels

Pregnancy is counted in weeks, but months aren't exactly four weeks. Here's why it's 40 weeks, how weeks map to months and trimesters, and a quick reference.

Why weeks, not months

Doctors track pregnancy in weeks because it's more precise. Dated from the first day of the last period, a full-term pregnancy runs 40 weeks, and each week brings specific developments and check-ups that a vaguer 'month' would blur. Weeks make it easy to say exactly how far along someone is and to time scans and tests. Months are how people talk about it socially, but weeks are the medical language.

Weeks to months isn't four

The tempting shortcut — four weeks to a month — doesn't quite work, because most months are a little longer than four weeks. On average a month is about 4.3 weeks, so 40 weeks comes to roughly 9.2 months, which is why pregnancy is famously 'nine months' and a bit. To convert, divide the number of weeks by 4.3: 20 weeks is about 4.6 months, 30 weeks about 7. The mismatch is small each month but adds up across the whole pregnancy.

The three trimesters

Because months are awkward, pregnancy is usually divided into three trimesters instead. The first runs from week 1 to 13, the second from 14 to 27, and the third from 28 to 40. Each spans roughly three months and marks a distinct phase — early development, steady growth, and the run-up to birth. Trimesters are the natural middle ground: broader than weeks, but cleaner than trying to force pregnancy onto the calendar's uneven months.

A quick reference

For everyday conversion, a few anchor points do the job. Around 12 weeks you're about 3 months and finishing the first trimester; 20 weeks is roughly 4.5 months and the mid-point, often when the anatomy scan happens; 28 weeks is about 7 months as the third trimester begins; and 40 weeks is the full nine months. Keep these in mind and you can translate between the medical 'weeks' and the everyday 'months' without any arithmetic.

Pregnancy week calculatorFind your current pregnancy week and trimester from your last period.Try the tool →

Frequently asked questions

How many months is 30 weeks pregnant?: About 7 months, early in the third trimester.
Why is pregnancy 40 weeks but nine months?: Because a month averages about 4.3 weeks, so 40 weeks is roughly 9.2 months.
When does each trimester start?: The first is weeks 1–13, the second 14–27 and the third 28–40.

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This is general information, not medical advice. Follow the dating and guidance of your doctor or midwife, which take precedence over any general chart.

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