A full-term pregnancy lasts 40 weeks, which is about 9 months (9 months and roughly 1 week by the calendar). Doctors count in weeks because it is more precise, but friends, family, forms and employers usually ask for months. A month is a little longer than four weeks, so the conversion is not simply weeks divided by 4. Use the chart below for a fast answer, or enter your dates in the due date calculator to get your exact week, trimester and due date.
Quick Answer: Pregnancy Weeks to Months Chart
| Month | Weeks | Trimester |
|---|---|---|
| Month 1 | Weeks 1–4 | First |
| Month 2 | Weeks 5–8 | First |
| Month 3 | Weeks 9–13 | First |
| Month 4 | Weeks 14–17 | Second |
| Month 5 | Weeks 18–22 | Second |
| Month 6 | Weeks 23–27 | Second |
| Month 7 | Weeks 28–31 | Third |
| Month 8 | Weeks 32–35 | Third |
| Month 9 | Weeks 36–40 | Third |
Read the chart as the month you are in. At 20 weeks you are in your fifth month, which means you have completed four. Month boundaries are approximate and differ slightly between sources. Trimester dates follow ACOG: the first trimester runs through 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks until birth.
How Many Months Pregnant Am I? Week-by-Week Conversions
Find your week below. “Exact months” divides your weeks by the average length of a calendar month (4.35 weeks), and “Month you are in” follows the chart above.
| Weeks pregnant | Exact months | Month you are in | Trimester |
|---|---|---|---|
| 4 weeks | 0.9 months | Month 1 | First |
| 6 weeks | 1.4 months | Month 2 | First |
| 8 weeks | 1.8 months | Month 2 | First |
| 10 weeks | 2.3 months | Month 3 | First |
| 12 weeks | 2.8 months | Month 3 | First |
| 13 weeks | 3.0 months | Month 3 | First |
| 14 weeks | 3.2 months | Month 4 | Second |
| 16 weeks | 3.7 months | Month 4 | Second |
| 18 weeks | 4.1 months | Month 5 | Second |
| 20 weeks | 4.6 months | Month 5 | Second |
| 22 weeks | 5.1 months | Month 5 | Second |
| 24 weeks | 5.5 months | Month 6 | Second |
| 26 weeks | 6.0 months | Month 6 | Second |
| 28 weeks | 6.4 months | Month 7 | Third |
| 30 weeks | 6.9 months | Month 7 | Third |
| 32 weeks | 7.4 months | Month 8 | Third |
| 34 weeks | 7.8 months | Month 8 | Third |
| 36 weeks | 8.3 months | Month 9 | Third |
| 37 weeks | 8.5 months | Month 9 | Third |
| 38 weeks | 8.7 months | Month 9 | Third |
| 39 weeks | 9.0 months | Month 9 | Third |
| 40 weeks | 9.2 months | Month 9 | Third |
For more detail on a specific week, see 13 weeks, 21 weeks and 32 weeks, or browse the week-by-week pregnancy hub.
Why Weeks and Months Do Not Line Up Neatly
Pregnancy is dated from the first day of your last menstrual period (LMP), which is about two weeks before conception. That means weeks 1 and 2 are counted even though you are not yet pregnant, and a positive home test often arrives around week 4. An average calendar month is about 4.35 weeks, so the same 40 weeks can be expressed three ways:
- Four-week months: 40 ÷ 4 = 10 months (often called “lunar” months).
- Calendar months: 40 ÷ 4.35 = about 9.2 months, or 9 months and roughly 1 week.
- Days: 280 days ÷ 30.4 days per month = about 9.2 months.
None of these is wrong, which is why you hear both “nine months” and “ten months” for the same pregnancy. For how dating works, see how doctors calculate gestational age and our gestational age calculator.
How to Calculate Your Pregnancy Months Yourself
- Find your gestational age in weeks from your due date or first ultrasound, not from the conception date.
- Divide your weeks by 4.35, the average number of weeks in a month.
- The whole number is the months you have completed. Add 1 for the month you are currently in.
Example: 26 weeks ÷ 4.35 is about 6.0, so you have completed roughly 6 months and are finishing month 6. At 18 weeks, 18 ÷ 4.35 is about 4.1, so you have completed 4 months and are in month 5.
Weeks, Months and Trimesters: How They Fit Together
Trimesters are the simplest way to group pregnancy because they are defined in weeks, not months. Explore the first trimester, second trimester and third trimester guides, or use the pregnancy countdown calculator to see how many weeks and days remain.
Early Term, Full Term and Late Term: What 40 Weeks Really Means
ACOG divides the end of pregnancy into named stages, so a due date is better understood as the middle of a window than a deadline:
- Early term: 37 weeks 0 days to 38 weeks 6 days
- Full term: 39 weeks 0 days to 40 weeks 6 days
- Late term: 41 weeks 0 days to 41 weeks 6 days
- Post-term: 42 weeks 0 days and beyond
Only a small share of babies, roughly 4–5%, arrive exactly on the due date. Your provider may also adjust your due date after an early ultrasound, which can shift your week and month counts by a few days. See gestational age vs fetal age if your conception date and your due date seem two weeks apart.
Should You Use Weeks or Months?
Use weeks with your doctor, midwife, ultrasound technician and pregnancy apps, because screening tests, vaccines and care milestones are scheduled by week. Months are fine for casual conversation and for forms, such as maternity leave paperwork, that ask for them.
Frequently Asked Questions
How many months is 40 weeks pregnant?
About 9 months, or roughly 9 months and 1 week using calendar months. If you count in four-week months, 40 weeks is 10 months.
Is pregnancy 9 months or 10 months?
Both are used. Forty weeks is 9 calendar months plus about a week, or ten four-week “lunar” months. Your due date is the same either way.
How many months is 20 weeks pregnant?
About 4½ to 5 months. By the formula 20 weeks is 4.6 months, and the chart puts you in month 5. It is the halfway point, and many people have their mid-pregnancy anatomy scan between 18 and 22 weeks.
How many months is 12 weeks pregnant?
About 3 months (2.8 by the formula). At 12 weeks you are near the end of the first trimester, which runs through 13 weeks 6 days.
How many months is 28 weeks pregnant?
About 6½ months (6.4 by the formula). Week 28 is the start of the third trimester and the start of month 7 on the chart.
How many months is 36 weeks pregnant?
About 8⅓ months (8.3 by the formula), which is month 9 on the chart. Babies born before 37 completed weeks are considered preterm, so contact your provider promptly if you notice labor signs at this stage.
How many weeks is the third trimester?
From 28 weeks until birth, roughly 12–13 weeks if you deliver at 40 weeks.
Why does pregnancy start before conception?
Because the date of conception is hard to pin down, doctors count from the first day of your last period, which is almost always known. Ovulation and conception usually happen about two weeks later, so a 40-week pregnancy is about 38 weeks from conception.
Skip the Math
Enter your last period or conception date in the due date calculator and get your exact week, trimester and due date instantly.
Sources
This page is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your OB-GYN or midwife about your own pregnancy.
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