How Many Weeks Pregnant Am I?
Reviewed by Safi Allah Khan
Medically reviewed by: Safi Allah Khan · Written by the Momi team · Last updated 20 October 2026
Quick answer: How many weeks pregnant am I? Count the full weeks since the first day of your last period. If your period started 15 weeks and 1 day ago, you are 15 weeks 1 day pregnant (15+1), in the second trimester. If you know your due date, count back 280 days to find your start date. The calculator does this instantly.
Enter the first day of your last period or your due date in the calculator above to see your week, day, trimester and how many days are left.
How the pregnancy week calculator works
The calculator uses gestational age, the same measure your doctor or midwife uses. Gestational age starts on the first day of your last menstrual period (LMP), about two weeks before conception in a typical 28-day cycle.
If you enter your last period: weeks pregnant = days since LMP ÷ 7 (the remainder is the extra days)
If you enter your due date: pregnancy start date = due date − 280 days, then count from that date to today
Worked example (today is 20 October 2026):
- Last period started 6 July 2026 → 106 days ago → 15 weeks 1 day pregnant
- Estimated due date: 12 April 2027, which is 174 days away
- Due date of 1 February 2027 → start date 27 April 2026 → 25 weeks 1 day pregnant
The calculator also shows your trimester and your baby's typical size this week, using the same data as our pregnancy week by week chart.
Weeks, months and trimesters
| Weeks pregnant | Month (approx.) | Trimester |
|---|---|---|
| 1–4 | 1 | First |
| 5–8 | 2 | First |
| 9–13 | 3 | First |
| 14–17 | 4 | Second |
| 18–22 | 5 | Second |
| 23–27 | 6 | Second |
| 28–31 | 7 | Third |
| 32–35 | 8 | Third |
| 36–40 | 9 | Third |
Months are approximate because 40 weeks is a little over 9 calendar months. That's why providers and apps count in weeks.
How accurate is it?
The arithmetic is exact. The accuracy depends on the date you enter:
- From your last period: good if your cycles are regular and close to 28 days and you're sure of the date. Long or irregular cycles can make it a week or more off.
- From your due date after a dating scan: the most reliable. ACOG says a first-trimester ultrasound (up to 13 weeks 6 days) is the most accurate way to establish gestational age, to within about 5 to 7 days.
- From a conception date: possible, but conception is hard to pin down. Our guide on the due date calculator from conception explains why.
If your dating scan gives a different date, use it. The NHS notes the 12-week scan can estimate how far along you are more accurately than your period date.
What to do with your week
- Read this week's page. Each week in our pregnancy week by week guide covers your baby's size, common symptoms and when to call your provider.
- Check which appointments are coming. Our pregnancy appointment schedule lists visits, scans and tests by week.
- Need a due date first? Use the due date calculator, which also handles conception and IVF dates.
Let Momi count the weeks for you
Instead of recalculating each week, the Momi app keeps count from your due date or last period. The home screen shows a progress ring with your current week and the weeks to go, plus your baby's size. Each week brings updates on your baby's development and tips, and you can keep a diary, bump photos and appointments alongside.
Download Momi free on the App Store or Google Play. Free with ads; optional Premium.
Sources
- ACOG Committee Opinion No. 700, Methods for Estimating the Due Date
- NHS, Due date calculator
- ACOG, Changes During Pregnancy
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 is the pregnancy week calculated?
Your pregnancy week is the number of full weeks since the first day of your last period. If you know your due date instead, the calculator counts back 280 days to find that start date and then counts forward to today.
How many weeks pregnant am I if I conceived two weeks ago?
About 4 weeks. Pregnancy is dated from your last period, which is usually about two weeks before conception, so add two weeks to the time since you conceived.
Is 12 weeks and 3 days the 12th or 13th week?
It's the 13th week. 12+3 means 12 completed weeks plus 3 days, so you are 12 weeks pregnant and in your 13th week. Most apps, scans and providers use the completed-weeks number.
How many months pregnant is 20 weeks?
20 weeks is about 4.5 months, usually described as the fifth month. Because 40 weeks don't divide evenly into 9 months, sources differ slightly, so weeks are the standard measure.
What if my scan says a different week?
Use the scan. ACOG says an ultrasound in the first trimester is the most accurate way to date a pregnancy. Enter the due date from your scan into this calculator to get your current week.