Weeks to Months Pregnant Converter
Type your week number (or your due date) and instantly see which month you are in, your trimester, how many days remain, and the full conversion chart from 4 to 42 weeks.
Your details
Providers track pregnancy in weeks and days (for example, 24+3 = 24 weeks 3 days) because day-level precision matters for screening windows. Months are for conversations; weeks are for care.
Type "20 weeks pregnant in months" into any forum and you will find the same confused thread a thousand times over, because pregnancy math runs on two calendars that refuse to align: doctors count weeks (40 of them), everyone else counts months (9 of them, or 10, depending on who you ask). The converter exists because the naive division is wrong — 40 weeks is not ten months of four weeks, because only February is ever four weeks long.
The conversion most clinics use is 4.3 weeks per month: divide your week count by 4.3 and you get the calendar-equivalent month. By that math, 20 weeks is about 4.6 months — well into the fifth month, past halfway. Trimester boundaries follow the same logic: the first trimester ends at 13 weeks 3 days, the second at 27 weeks 1 day, and "nine months pregnant" in the everyday sense means somewhere around 36–40 weeks.
This converter also settles the nine-versus-ten-months argument honestly. Counting only completed lunar (4-week) months gives ten; counting calendar months from your last period gives roughly nine and a third. Both are "right" — they are just different rulers. We show both so you can use whichever matches the conversation you are having.
How the calculation works
Months = weeks ÷ 4.345, the average length of a calendar month. The converter also computes your trimester from the clinical boundaries (13w3d and 27w1d), the days remaining to 40 weeks, and — if you entered a due date instead — derives your week count as 40 minus the days remaining.
The full chart below the result lists every week from 4 to 42 with its month and trimester, so you can look up any week you encounter — a relative's, a book's, or your own from a previous pregnancy.
Frequently asked questions
How many months is 20 weeks pregnant?
About 4.6 months by the standard 4.3-weeks-per-month conversion — meaning you are in your fifth month and just past halfway through pregnancy. The common mistake is dividing by 4 (getting "5 months") which treats every month as 28 days; real calendar months average 30.4 days, which is where the 4.3 figure comes from.
Is pregnancy 9 or 10 months long?
Both answers come from legitimate counting methods. Forty weeks contains ten 4-week "lunar" months, but measured in real calendar months from the last period, a full-term pregnancy runs about nine and a third months. When someone says "nine months pregnant," they usually mean roughly 36–40 weeks — the ninth calendar month of pregnancy.
When does each trimester start and end?
The boundaries most clinicians use: first trimester weeks 1 through 13+3, second trimester 13+3 through 27+1, third trimester 27+1 through birth. You will also see 13/26/40 rounded versions in apps and books; the day-level boundaries matter mainly for interpreting screening windows, where "13 weeks 6 days" is a hard cutoff for some tests.
What week does month 6 of pregnancy start?
Using 4.3 weeks per month, month 6 begins at about 22 weeks (5 × 4.3 = 21.5). The cleanest mental map: month 1 = weeks 1–4.3, month 2 = weeks 4.3–8.6, month 3 = 8.6–13, month 4 = 13–17, month 5 = 17–21.5, month 6 = 21.5–26, month 7 = 26–30, month 8 = 30–34.5, month 9 = 34.5–40. Boundary weeks land differently depending on the source, so treat week-level numbers as exact and month labels as approximate.
Why do doctors use weeks instead of months?
Because everything in prenatal care keys off the week count: the NT scan window (11w0d–13w6d), the anatomy scan (18–22 weeks), the glucose screen (24–28 weeks), and fetal growth curves are all defined in weeks. Months are too coarse to schedule anything by — the difference between 12 weeks and 13 weeks is the difference between an open and a closed screening window.
References & sources
- ACOG Committee Opinion 700: Methods for Estimating the Due Date (gestational age conventions).
- ACOG guidance on trimester definitions (13w3d / 27w1d boundaries).
- IOM/NAM pregnancy duration conventions in Weight Gain During Pregnancy (2009).