How many months is 20 weeks pregnant?

About 5 months — the middle of the second trimester. Twenty weeks is roughly the halfway point of pregnancy.

Why is pregnancy 40 weeks but only 9 months?

Because a month averages about 4.35 weeks, not 4. Forty weeks divided by 4.35 is about 9.2 months, which the prenatal convention rounds into 9 months.

How many weeks is the third trimester?

It starts at 28 weeks (month 7) and runs to birth, around 40 weeks.

Is 9 months or 10 months more accurate?

Nine months is the convention. It only looks like 10 if you assume exactly four weeks per month, which overcounts.

Pregnancy & Baby

Pregnancy Weeks to Months

Pregnancy is tracked in weeks, but people ask about it in months. Enter your week to see which month of pregnancy you are in and the trimester — using the standard prenatal convention.

Your numbers

Result

Month of pregnancy

Trimester: Second
In decimal months: 4.6

Weeks to months (prenatal convention)
Weeks	Month	Trimester
1–4	1	First
5–8	2	First
9–13	3	First
14–17	4	Second
18–22	5	Second
23–27	6	Second
28–31	7	Third
32–35	8	Third
36–40	9	Third

Why weeks and months don’t divide evenly

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 standard mapping

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".

Trimesters

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.

Informational and educational result. Not a substitute for professional advice.

Frequently asked questions

Sources

Prenatal week-to-month convention (40 weeks / 9 months)

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