Pregnancy Weeks to Months Converter
Convert pregnancy weeks and days into months and identify your trimester, using either calendar months or four-week 'lunar' months — the two conventions that cause confusion.
Last updated: May 2026
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About this calculator
The reason 'how many months pregnant am I?' is so confusing is that two different month conventions are both in common use, and they disagree by almost a full month over a pregnancy. Pregnancy is measured precisely in weeks (40 weeks at term), but people instinctively think in months. The first convention is calendar months: a calendar month averages 30.44 days (365.25 ÷ 12), so you convert total days to months by dividing by 30.44. Under this convention 40 weeks (280 days) is about 9.2 months — the familiar 'nine months.' The second convention is lunar or 'pregnancy' months, where each month is treated as exactly four weeks; under this scheme 40 weeks is exactly 10 months, which is why some sources say pregnancy lasts ten months. Neither is wrong — they answer the same question with different rulers. This converter takes your completed weeks plus any extra days, converts to total days, and divides by the ruler you choose, then labels your trimester. Trimesters are defined by week: the first trimester is weeks 1–13, the second is weeks 14–27, and the third is weeks 28 to birth. All three inputs are used — weeks and days set the elapsed gestation, and the month-convention selector decides which divisor to apply — so you get a months figure that matches the convention your doctor, app, or book is using.
How to use
Worked example. You are 20 weeks and 0 days pregnant and want the answer in calendar months. Step 1 — total days: 20 × 7 + 0 = 140 days. Step 2 — divide by the calendar-month length: 140 ÷ 30.44 = 4.6 months. Step 3 — determine the trimester from weeks: 20 is between 14 and 27, so the second trimester. The result reads '4.6 months (2nd trimester).' Switch the convention to lunar months and the same 140 days becomes 140 ÷ 28 = 5.0 months — half a month higher — which is exactly the discrepancy that makes people think they are further along under one system than another. The trimester label stays the same because it is defined by weeks, not months: you are solidly mid-pregnancy either way.
Frequently asked questions
How many months is 20 weeks pregnant?
It depends on which month you mean. Using calendar months (about 30.44 days each), 20 weeks is 140 days ÷ 30.44 ≈ 4.6 months. Using four-week 'lunar' months, it is 140 ÷ 28 = 5.0 months exactly. Both are correct — they just use different rulers, which is why apps and books sometimes disagree by half a month. Either way, 20 weeks puts you in the second trimester and right around the midpoint of a 40-week pregnancy. This converter lets you pick the convention so your months figure matches the source you're comparing against.
Why does pregnancy last 9 months but also 40 weeks (10 months)?
Because of the two month conventions. Forty weeks is 280 days. Divided by an average calendar month of 30.44 days, that's about 9.2 months — the everyday 'nine months.' But if you define a month as exactly four weeks (a lunar month), 40 weeks is precisely 10 months. So pregnancy is simultaneously 'nine months' by the calendar and 'ten months' by four-week counting. Neither is wrong; weeks are the unambiguous unit clinicians actually use, which is why due dates and milestones are always given in weeks rather than months.
What week does each trimester of pregnancy start?
Trimesters are defined by gestational week, not by months. The first trimester runs from week 1 through the end of week 13, the second trimester from week 14 through the end of week 27, and the third trimester from week 28 until birth (typically around week 40). Because trimesters are counted in weeks, your trimester doesn't change when you switch between calendar and lunar month conventions. This converter reports your trimester directly from your completed weeks so it stays consistent regardless of how you express your progress in months.