# Pregnancy Weeks to Months Calculator _Turn weeks pregnant into months, or months into weeks, with a table for every week from 1 to 42._ - Source: https://proba.baby/en/tools/pregnancy-weeks-to-months-calculator - Language: English - Last verified: v1.0.4 · 7 Sep 2026 - Last updated: 2026-10-10 ## How it works Choose how many weeks pregnant you are, and add days if you know them. The calculator shows which month of pregnancy that is, how many calendar months it equals, and which trimester you are in. To go the other way, switch to months to weeks and pick a month. Weeks are counted from the first day of your last period, the way a midwife or doctor dates a pregnancy. That is why a full pregnancy is 40 weeks, even though the baby has been growing for about 38 of them. Months are harder than weeks, because a month is not a fixed number of weeks. Most pregnancy sites, including What to Expect and The Bump, use one table: month 1 is weeks 1 to 4, month 2 is weeks 5 to 8, month 3 is weeks 9 to 13, and so on up to month 9 at weeks 36 to 40. Weeks 41 and 42 fall after month 9. This calculator uses that table, so its answer matches what you will read elsewhere. Some people mean something simpler: how many calendar months have passed. For that, the calculator also gives the calendar figure, which is the number of days divided by 30.44, the average length of a month. At 20 weeks that is about 4.6 months, while the table calls it the start of month 5. Neither is wrong; they answer slightly different questions. Under the calculator you will find a full table from week 1 to week 42, so you can look up any week without typing. For what is happening at your week, see the pregnancy week-by-week guide linked below. ## The math Month of pregnancy comes from the completed week. Week 13 with 6 extra days is still week 13, so it is still month 3. The bands are: month 1 is weeks 1 to 4, month 2 is 5 to 8, month 3 is 9 to 13, month 4 is 14 to 17, month 5 is 18 to 22, month 6 is 23 to 27, month 7 is 28 to 31, month 8 is 32 to 35 and month 9 is 36 to 40. Calendar months = (weeks × 7 + days) ÷ 30.44, rounded to one decimal. 30.44 is the average month in the Gregorian calendar (365.25 ÷ 12). Example: 20 weeks is 140 days, and 140 ÷ 30.44 is 4.6 months. Months to weeks, by the calendar, is months × 30.44 ÷ 7. Five calendar months is about 21.7 weeks. The table version is simpler: month 5 covers weeks 18 to 22. Trimesters follow the usual cut-offs: first is weeks 1 to 13, second is 14 to 27, third is 28 onward. The days to 40 weeks count back from 280 days. ## FAQ **How many months is 20 weeks pregnant?** 20 weeks pregnant is month 5 in the table most pregnancy sites use (month 5 is weeks 18 to 22). By the calendar it is about 4.6 months, because 140 days divided by 30.44 is 4.6. **Is pregnancy 9 months or 10 months?** A full pregnancy is 40 weeks. In the usual table that is 9 months, and by the calendar it is about 9.2 months. The "10 months" you sometimes hear counts 4-week months: 40 weeks divided by 4 is 10. **How many weeks are in a month of pregnancy?** It varies from 4 to 5 weeks. In the table most sources use, months 1, 2, 4, 7 and 8 are 4 weeks each, and months 3, 5, 6 and 9 are 5 weeks each. The calendar average is 4.35 weeks a month. **How many months is 12 weeks pregnant?** 12 weeks pregnant is month 3 in the usual table (weeks 9 to 13), and about 2.8 calendar months. You are still in the first trimester, which runs to week 13. **Why do other sites give a different number of months?** Sites choose between two methods: a week table like the one here, or dividing weeks by 4 or by 4.35. The answers can differ by about a month. Both are correct for what they measure. This calculator shows the table month and the calendar figure together so you can compare. **Which trimester am I in?** The first trimester is weeks 1 to 13, the second is weeks 14 to 27, and the third is week 28 until birth. The calculator shows your trimester next to the month. ## Sources - ACOG Committee Opinion No. 700: Methods for Estimating the Due Date — Pregnancy is dated in weeks from the first day of the last menstrual period; a full pregnancy is 280 days: https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date - NHS: Pregnancy week by week — Weeks, and the three trimesters, as the NHS counts them: https://www.nhs.uk/pregnancy/week-by-week/ - The Bump: Pregnancy weeks to months — The week-to-month table this tool follows (also used by What to Expect): https://www.thebump.com/a/pregnancy-weeks-to-months --- ProbaBaby (listed on the App Store as “Pregnancy & Baby Tracker - PB”) is a free iPhone, iPad and Apple Watch app for pregnancy, newborn and postpartum tracking, built for twins, in 32 in-app languages, with an optional AI baby-face prediction that computes Mendelian genetics (Punnett squares) from the traits you enter before any image is generated. Tracking, tools, PDF reports and family sharing are free; only AI predictions use credits — 5 starter credits cover your first Scientific prediction — and no subscription is required. Requires iOS 26 or later. Not available on Android.