weeks ↔ months converter

Pregnancy Weeks to Months Converter

Convert pregnancy weeks to months and trimesters instantly, with a full week-by-week table. See which month 12, 20, 28 or 36 weeks is and why pregnancy is counted in weeks.

The short answer: Pregnancy is usually described as nine months: month 1 is weeks 1–4, month 2 weeks 5–8, month 3 weeks 9–13, month 4 weeks 14–17, month 5 weeks 18–22, month 6 weeks 23–27, month 7 weeks 28–31, month 8 weeks 32–35 and month 9 weeks 36–40. Weeks are counted from the first day of your last period.

Pregnancy weeks to months table

Months, weeks and trimesters
Month	Weeks	Trimester	Week guides
Month 1	1–4	First	1 · 2 · 3 · 4
Month 2	5–8	First	5 · 6 · 7 · 8
Month 3	9–13	First	9 · 10 · 11 · 12 · 13
Month 4	14–17	Second	14 · 15 · 16 · 17
Month 5	18–22	Second	18 · 19 · 20 · 21 · 22
Month 6	23–27	Second	23 · 24 · 25 · 26 · 27
Month 7	28–31	Third	28 · 29 · 30 · 31
Month 8	32–35	Third	32 · 33 · 34 · 35
Month 9	36–40	Third	36 · 37 · 38 · 39 · 40

Why months and weeks never quite match

A full-term pregnancy is counted as 40 weeks, or 280 days, from the first day of your last menstrual period. Twelve calendar months average about 4.35 weeks each, so 40 weeks is roughly 9 calendar months and 1 week. Different books split the weeks into months slightly differently; the table above is the most common convention and keeps each trimester boundary (13 and 27 weeks) at the end of a month.

For anything medical — scans, blood tests, whether a baby is term — your care team will use weeks and days. ACOG defines full term as 39 weeks 0 days to 40 weeks 6 days, with early term from 37 weeks and late term from 41 weeks.

Quick answers for common weeks

8 weeks — second month, first trimester.
12 weeks — third month; many people have their dating scan around now.
16 weeks — fourth month, second trimester.
24 weeks — sixth month.
28 weeks — seventh month and the start of the third trimester.
32 weeks — eighth month.
36 weeks — ninth month; the final stretch.

Not sure how many weeks you are? Start with the due date calculator.

Common questions

How many months is 40 weeks pregnant?

Forty weeks is the end of the ninth month. Because most months are a little longer than four weeks, 40 weeks is about 9 calendar months and one week from the first day of your last period — which is why people argue about whether pregnancy is nine or ten months.

Is pregnancy 9 months or 10 months?

Counted in calendar months from the first day of the last period, 40 weeks is about 9¼ months. Counted as blocks of exactly four weeks it would be ten “months”. Most people and most pregnancy references use nine months with the table on this page.

How many weeks are in each trimester?

The first trimester is weeks 1 to 13, the second is weeks 14 to 27 and the third is week 28 until birth.

How many months is 20 weeks pregnant?

Twenty weeks is in the fifth month and marks the halfway point of a 40-week pregnancy.

Why do doctors use weeks instead of months?

Weeks are precise and the same length every time, while months vary from 28 to 31 days. Scans, tests and baby development are all timed in weeks and days, such as “12 weeks and 3 days”.

Sources & editorial process

Written by the Elara editorial team from the sources above. This page has not been reviewed by an independent clinician. Read how we write and update health pages. Last updated 27 September 2026.

Medical disclaimer: this page is general education, not medical advice, diagnosis or treatment. If you are worried about yourself or your baby, contact your midwife, doctor or maternity unit straight away. In an emergency call your local emergency number.