Pregnancy weeks to months: the complete conversion guide

Pregnancy is tracked in weeks by clinicians but talked about in months by nearly everyone else — which is why “how many months is 20 weeks?” is one of the most common questions in early pregnancy. The short version: weeks are the precise measure, months are a friendly approximation, and the two don’t line up as neatly as you’d expect. Our weeks to months converter does the sums instantly; this guide explains the logic behind them.

Why weeks and months don’t divide evenly

The obvious guess — four weeks to a month — would make pregnancy ten months long (40 ÷ 4). But a calendar month averages about 30.4 days, not 28, so it’s a bit longer than four weeks. Across a whole pregnancy that gap compounds: 40 weeks is roughly 9.2 calendar months, which is why pregnancy is described as nine months even though the simple division suggests ten. This is the single most common source of confusion, and it’s just arithmetic, not anything medical.

The standard weeks-to-months chart

Because the boundaries are uneven, obstetric convention maps the 40 weeks onto nine months like this:

• Month 1: weeks 0–4½ — first trimester
• Month 2: weeks 4½–9 — first trimester
• Month 3: weeks 9–13½ — first trimester
• Month 4: weeks 13½–18 — second trimester begins around here
• Month 5: weeks 18–22½ — second trimester
• Month 6: weeks 22½–27 — second trimester
• Month 7: weeks 27–31½ — third trimester begins around here
• Month 8: weeks 31½–36 — third trimester
• Month 9: weeks 36–40 — third trimester

Different charts nudge these boundaries by a few days, so don’t treat the half-weeks as exact — they’re a convention for turning a continuous count into tidy months.

Quick answers to the common ones

• How many months is 12 weeks? Three months — the end of the first trimester.
• How many months is 16 weeks? Four months, early in the second trimester.
• How many months is 20 weeks? About five months — the halfway point.
• How many months is 28 weeks? Seven months, and the start of the third trimester.
• How many months is 36 weeks? Nine months — nearly there.

How months map onto trimesters

Grouped into trimesters, months 1–3 are the first trimester, months 4–6 the second, and months 7–9 the third — roughly, since the trimester lines (14 and 28 weeks) fall mid-month. To see the exact calendar dates of each trimester for your pregnancy, use the trimester calculator, and for the milestones within them, pregnancy trimesters and milestones.

Weeks, months — and your actual due date

Converting weeks to months tells you the stage; to get dates you need a starting point. From your last period, the gestational age calculator shows exactly how far along you are in weeks and days, and the full OB wheel adds your due date and milestone dates. Remember gestational age counts from the last period, about two weeks before conception — explained in gestational age vs. fetal age.

The takeaway

Four weeks isn’t quite a month, so pregnancy’s 40 weeks fold into nine months with slightly uneven joins. Use the chart above for a quick check, the converter for any specific week, and a dating calculator when you need real dates.