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Pregnancy Weeks to Months Calculator
Pregnancy Weeks
to Months Calculator
convert your weeks into months and trimesters
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How far along are you?
Completed weeks, then days — the way your app or scan report writes it.
- 4 Weeks
- 5 Weeks
- 6 Weeks
- 7 Weeks
- 8 Weeks
- 9 Weeks
- 10 Weeks
- 11 Weeks
- 12 Weeks
- 13 Weeks
- 14 Weeks
- 15 Weeks
- 16 Weeks
- 17 Weeks
- 18 Weeks
- 19 Weeks
- 20 Weeks
- 21 Weeks
- 22 Weeks
- 23 Weeks
- 24 Weeks
- 25 Weeks
- 26 Weeks
- 27 Weeks
- 28 Weeks
- 29 Weeks
- 30 Weeks
- 31 Weeks
- 32 Weeks
- 33 Weeks
- 34 Weeks
- 35 Weeks
- 36 Weeks
- 37 Weeks
- 38 Weeks
- 39 Weeks
- 40 Weeks
- 0 Days
- 1 Day
- 2 Days
- 3 Days
- 4 Days
- 5 Days
- 6 Days
About this Calculator
Weeks and months never divide evenly, so this tool does the conversion the way clinical guidance defines it and shows your appointments alignment.
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What you enter?
Completed weeks and days, exactly as your app or scan report writes it, 21 weeks 3 days means 21 full weeks done.
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What you get?
Your pregnancy month, your trimester, how far through 40 weeks you are, and roughly how long is left.
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How it's worked out?
Months come from the day count, not a rough divide by four — so 40 weeks lands on nine months and a week, as it should.
Your Pregnancy Week
by Week
Pregnancy weeks to months
Months vary in length, so pregnancy uses week ranges. Match your weeks + days to a row.
1st Month
Weeks 1 - 4
2nd Month
Weeks 5 - 8
3rd Month
Weeks 9 - 13
4th Month
Weeks 14 - 17
5th Month
Weeks 18 - 22
6th Month
Weeks 23 - 27
7th Month
Weeks 28 - 31
8th Month
Weeks 32 - 35
9th Month
Weeks 36 - 40
Trimesters at a glance
Weeks in Each Trimester?
Each stage is roughly 13 to 14 weeks. Here's what happens in each, and the checks that come with it.
What's next?
Pick up where your month leaves off.
Recommended Topics
Recommended Topics for you
Reproductive health
Pregnancy Journey
Age & Fertility
Early Pregnancy
Preconception Health
Getting Pregnant
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How Many Months Pregnant Are You?
Quick Answer: Pregnancy is counted in 40 weeks, which is about 9 calendar months. To convert: weeks 1 to 13 are months 1 to 3 (first trimester), weeks 14 to 27 are months 4 to 6 (second trimester), and weeks 28 to 40 are months 7 to 9 (third trimester). Enter your weeks and days above for your exact month and trimester.
How Do You Use This Weeks to Months Calculator?
- Enter what you know: your completed weeks and days, exactly as your doctor or your scan report writes them.
- Get your exact position: which month you are in, your trimester, how far along you are and how much time is left.
- Jump to your week: the result takes you to that week in the week-by-week guide, with the baby's size for it.
What Is a Pregnancy Weeks to Months Calculator?
Quick Answer: It is a converter between the two languages of pregnancy. Doctors count in weeks from your last period, while family and month-wise traditions count in months. The calculator turns "18 weeks" into "5th month, second trimester" so both conversations make sense.
The weeks system is the medical standard because every scan, test and milestone is week-specific, as the ACOG guide to fetal growth and the NHS week-by-week guide both show. Months are what everyone around you speaks. You need both, and the mapping between them is fixed rather than a matter of opinion, which is exactly what the chart below settles.
How Do You Convert Pregnancy Weeks to Months? (Chart)
Quick Answer: Use the standard mapping below. Note that months 3, 5, 6 and 9 span 5 weeks, not 4, which is why dividing your weeks by four gives the wrong month from mid-pregnancy onward.
| Month | Weeks | Trimester | Common milestone |
|---|---|---|---|
| Month 1 | Weeks 1 to 4 | First | Missed period, positive test |
| Month 2 | Weeks 5 to 8 | First | First scan, heartbeat |
| Month 3 | Weeks 9 to 13 | First | NT scan, nausea peaks and eases |
| Month 4 | Weeks 14 to 17 | Second | Energy returns |
| Month 5 | Weeks 18 to 22 | Second | Anomaly scan, first kicks |
| Month 6 | Weeks 23 to 27 | Second | Glucose test, viability week (24) |
| Month 7 | Weeks 28 to 31 | Third | Kick counting starts (28) |
| Month 8 | Weeks 32 to 35 | Third | Growth scan, bag packing |
| Month 9 | Weeks 36 to 40 | Third | Full term (37+), due date (40) |
Mapping per ACOG and NHS week groupings. Trimesters: first = weeks 1 to 13, second = weeks 14 to 27, third = weeks 28 to 40.
Why Is Pregnancy 40 Weeks but Only 9 Months?
Because a calendar month is 4.3 weeks, not 4. Forty weeks equals about 9 calendar months plus a few days, but exactly 10 lunar months of 4 weeks. Both "9 months" and "10 months" are correct in their own counting system, which is precisely why doctors stick to weeks.
There is a second quirk: the count starts from the first day of your last period, about two weeks before conception. So at "4 weeks pregnant" (the missed period), the embryo is roughly two weeks old. You are never as far along as the week number suggests, and that is normal for everyone.
Which Month Are You In? Running Month vs Completed Months
When your family asks "which month", they mean the running month: the month you are currently in. When a chart or app says "4 months pregnant", it usually means completed months. At 18 weeks, both are true at once: 4 months completed, 5th month running. This calculator shows both so no conversation goes wrong.
This distinction causes real confusion in Indian households, where "paanchva mahina chal raha hai" (the fifth month is running) is the standard phrasing, while English apps and charts report completed months. Neither is wrong; they are two conventions describing the same week. When in doubt, say the week number and let this page translate.
How Are Pregnancy Weeks Actually Counted?
From the first day of your last menstrual period (LMP), not from conception. The due date is 40 weeks (280 days) from that day, per Naegele's rule. If your cycles are irregular or you do not know your LMP, the dating scan in the first trimester fixes the week count, and that scan date wins over the LMP estimate.
Also Read
Pregnancy Week by Week: Everything You Need to Know | Due Date Calculator | Calculating Your Due Date with Irregular Periods
References
American College of Obstetricians and Gynecologists (ACOG). How Your Fetus Grows During Pregnancy.
NHS. Pregnancy Week-by-Week.