Pregnancy Week Calculator
Enter the first day of your last period and see how many weeks and days pregnant you are, which trimester you’re in, your estimated due date and the time left. Everything is calculated in your browser.
Enter your last period date
Results update instantly.
Everything is calculated in your browser — no dates are uploaded or stored.
How the pregnancy week and due date are calculated
Pregnancy is counted from the first day of your last menstrual period (LMP), not from conception. That is because the exact day of conception is usually unknown, while the LMP is a clear starting point. A typical pregnancy lasts 40 weeks (280 days) from the LMP.
Step by step
- Count the days between today and your LMP, then divide by 7 — the whole number is the week, the remainder is the day (e.g. 87 days = 12 weeks 3 days).
- Find the trimester: weeks 1–12 are the first, 13–27 the second, 28–40 the third.
- Add 280 days to the LMP for the estimated due date.
If you know your conception date instead, add about 14 days to estimate the LMP; the tool does this for you on the conception tab.
Pregnancy calculator FAQ
Why is pregnancy counted from the last period, not conception?
The exact day of conception is rarely known, while the first day of your last period is a clear, datable event. So pregnancy weeks and the due date are counted from the LMP worldwide. Ovulation and conception usually happen about two weeks later.
How is the due date calculated?
The estimated due date is the LMP plus 280 days (40 weeks). For an LMP of 1 January, the due date is 8 October. It's an estimate — most babies arrive a few days before or after.
What does '12 weeks pregnant' mean?
It's the number of days between today and your last period, divided by 7. If 90 days have passed, you are 12 weeks and 6 days pregnant. The extra days beyond the full week are shown separately.
Can I calculate from the conception date?
Yes. If you know the conception date, use the conception tab; the tool adds about 14 days to estimate the LMP and works out the week from there.
When do the trimesters change?
Weeks 1–12 are the first trimester, 13–27 the second, and 28–40 the third. The tool shows which trimester you're in and a short note for the week.
Is the due date exact?
No, it's an estimate — only about 1 in 20 babies are born on the exact date. Your doctor may adjust it from an early ultrasound. This tool is not medical advice.