Is Pregnancy 9 or 10 Months? How 40 Weeks Convert to Months
Reviewed by Safi Allah Khan
Medically reviewed by: Safi Allah Khan · Written by the Momi team · Updated 20 October 2026
Quick answer: Is pregnancy 9 or 10 months? Both answers are right, depending on how you count. A full-term pregnancy is 40 weeks (280 days) from the first day of your last period. That's about 9 calendar months plus a week, or exactly 10 months if each month is 4 weeks. Doctors and midwives use weeks.
People say "nine months", but pregnancy apps and doctors talk in weeks, and 40 weeks doesn't divide neatly into months. This guide shows the maths, a weeks-to-months chart, and how trimesters fit in. To find your own week, try How Many Weeks Pregnant Am I?.
Why 40 weeks isn't exactly 9 months
The maths:
- 40 weeks = 280 days. This is the standard length of pregnancy that ACOG and the NHS use to estimate the due date, counted from the first day of your last period.
- An average calendar month is about 30.4 days (365.25 days ÷ 12).
- 280 ÷ 30.4 ≈ 9.2 months. So 40 weeks is about 9 calendar months and 6 or 7 days.
- 4 weeks × 10 = 40 weeks. If every "month" is 4 weeks, as in some books and apps, pregnancy is 10 months. These are sometimes called lunar months (28 days).
Counting from the last period vs conception
The 40-week count starts about 2 weeks before you conceive, because the last period is a date most people know. So by the time you conceive, you're already counted as about 2 weeks pregnant. See why pregnancy calculators add 2 weeks.
| Counting from | Length | In calendar months |
|---|---|---|
| First day of last period (standard) | 40 weeks / 280 days | About 9.2 months |
| Conception | About 38 weeks / 266 days | About 8.7 months |
| 4-week "months" | 40 weeks | 10 months |
Pregnancy weeks to months chart
There's no single official weeks-to-months chart. Here are two common ways to convert. The calendar-month column is the exact maths from your last period; the 4-week column is how many apps and books split pregnancy.
| Month | Calendar-month weeks (approx.) | Common app/book split | Trimester (ACOG) |
|---|---|---|---|
| 1 | 0 to 4 weeks + 2 days | Weeks 1–4 | First |
| 2 | to 8 weeks + 5 days | Weeks 5–8 | First |
| 3 | to 13 weeks | Weeks 9–13 | First |
| 4 | to 17 weeks + 3 days | Weeks 14–17 | Second |
| 5 | to 21 weeks + 5 days | Weeks 18–22 | Second |
| 6 | to 26 weeks + 1 day | Weeks 23–27 | Second |
| 7 | to 30 weeks + 3 days | Weeks 28–31 | Third |
| 8 | to 34 weeks + 6 days | Weeks 32–35 | Third |
| 9 | to 39 weeks + 1 day | Weeks 36–40 | Third |
| "Extra" | 39 weeks + 1 day to 40 weeks | Included in month 9 | Third |
So if someone tells you they're "6 months pregnant", they could be anywhere from about 22 to 27 weeks. That's why your midwife will always ask for your week. For a printable version, see the pregnancy week by week chart.
How trimesters fit in
Pregnancy is split into three trimesters of roughly three months each. ACOG's weeks:
- First trimester: up to 13 weeks + 6 days. See first trimester symptoms.
- Second trimester: 14 weeks to 27 weeks + 6 days. See when the second trimester starts.
- Third trimester: 28 weeks to birth. See the third trimester guide.
NHS week-by-week pages start the second trimester at week 13. The difference is only about rounding.
Many pregnancies last more or less than 40 weeks
40 weeks is the date your due date is based on, but it isn't a deadline. ACOG defines term like this:
| Weeks | Term |
|---|---|
| 37 weeks to 38 weeks + 6 days | Early term |
| 39 weeks to 40 weeks + 6 days | Full term |
| 41 weeks to 41 weeks + 6 days | Late term |
| 42 weeks and later | Post-term |
Only about 1 in 20 babies arrive on their due date. A baby born at 42 weeks has been growing for almost 9 calendar months and 3 weeks from the last period, so for some people pregnancy really does stretch towards 10 calendar months. Your provider will talk to you about options if you go past your due date.
Why weeks are more useful than months
- Development happens in weeks. Your baby's growth changes noticeably from week to week, as you'll see in baby size by week.
- Tests have windows. The dating scan, screening tests and the anatomy scan are booked by week. See the pregnancy appointment schedule.
- Months vary. February is 28 days and other months are 31, so "months pregnant" depends on when you started.
How Momi helps
Momi does the conversion for you. Enter your last period or due date and Momi shows your week and day, how many weeks to go, and a weekly guide with your baby's size, updated automatically as each day passes. Add your appointments to visits and note how you feel in the diary.
Download Momi free on the App Store or Google Play. Free with ads; optional Premium.
Related: due date calculator · how is the due date calculated · pregnancy week by week
Sources
- ACOG, Committee Opinion 700: Methods for Estimating the Due Date
- ACOG, How Your Fetus Grows During Pregnancy
- ACOG, Committee Opinion 579: Definition of Term Pregnancy
- ACOG, When Pregnancy Goes Past Your Due Date
- NHS, Due date calculator
- NHS, Your pregnancy week by week
Medical disclaimer: Momi and this website provide general information, not medical advice. Always talk to your doctor or midwife; in an emergency call your local emergency number.
FAQ
How many months is 40 weeks pregnant?
40 weeks is 280 days. An average calendar month is about 30.4 days, so 40 weeks is about 9.2 months: 9 calendar months plus about a week. If you count in 4-week months, 40 weeks is exactly 10 months.
How many weeks is 9 months pregnant?
Measured in calendar months from your last period, the 9-month mark falls at about 39 weeks. Many apps and books call weeks 36 to 40 the ninth month. Either way, you're in the final stretch and close to full term, which ACOG defines as 39 weeks.
Why do doctors count pregnancy in weeks instead of months?
Weeks are precise and months aren't. Months vary from 28 to 31 days, and baby development, scans and screening tests happen in windows of a few weeks. ACOG and the NHS both date pregnancy in weeks and days from the first day of your last period.
Is pregnancy really 9 months if you count from conception?
Counting from conception, a typical pregnancy is about 38 weeks, or 266 days, which is closer to 8.7 calendar months. The 40-week count starts about 2 weeks before conception, from the first day of your last period, which is why the numbers differ.
How many months is each trimester?
Each trimester is about 3 months. Using ACOG's weeks, the first trimester is up to 13 weeks 6 days, the second is 14 to 27 weeks 6 days, and the third is 28 weeks to birth. Because the third trimester runs to 40 weeks or beyond, it's often the longest.
Track this in Momi
Momi is a free pregnancy tracker app for iPhone and Android that follows pregnancy week by week, estimates the due date, and keeps a pregnancy journal, weight log, kick counter and contraction timer in one place.
