Due date calculator
Pick how you want to calculate, enter the date, and see your estimated due date, how far along you are today, and your trimester.
Based on ACOG guidance on estimating the due dateLast reviewed: October 9, 2026
The short answer
Your estimated due date is 280 days (40 weeks) after the first day of your last period, assuming a 28-day cycle; add or subtract the difference if your cycle is longer or shorter. From a known conception date add 266 days. For IVF, add 261 days to a day-5 transfer or 263 days to a day-3 transfer.
How it is calculated
From last period: due date = first day of LMP + 280 days + (cycle length − 28)
From conception: due date = conception date + 266 days
IVF: transfer date + 261 days (day-5 embryo) or + 263 days (day-3 embryo)
Weeks pregnant today = days since the (adjusted) LMP, shown as weeks and days.
Worked example
Emily's last period started on March 15, 2026, and her cycle is usually 32 days.
- The 280-day rule alone gives December 20, 2026
- Her cycle is 4 days longer than 28, so add 4 days: Thursday, December 24, 2026
- Second trimester starts June 25, 2026; third trimester October 1, 2026
- Full term (39 weeks) begins December 17, 2026
Emily's estimated due date: December 24, 2026.
Quick reference table
| Date entered | Days added | Due date |
|---|---|---|
| Last period, 28-day cycle | + 280 | November 8, 2026 |
| Last period, 35-day cycle | + 287 | November 15, 2026 |
| Last period, 24-day cycle | + 276 | November 4, 2026 |
| Conception date | + 266 | October 25, 2026 |
| IVF day-3 transfer | + 263 | October 22, 2026 |
| IVF day-5 transfer | + 261 | October 20, 2026 |
Special cases and common mistakes
- Cycle length moves the date. In a 28-day cycle ovulation is around day 14; it comes later in longer cycles and earlier in shorter ones, so the calculator adds or subtracts the difference from 28. With cycles under 21 or over 35 days, calendar-based dating is less reliable.
- An early ultrasound wins. ACOG says ultrasound measurement up to 13 weeks 6 days is the most accurate way to date a pregnancy. If your provider gives you a date from a scan, use that one.
- IVF dates come from the embryo. The due date is set from the embryo age and the transfer date: 261 days after a day-5 transfer, 263 days after a day-3 transfer.
- Week count starts before conception. Pregnancy is counted from the first day of the last period, so the first two weeks happen before you are actually pregnant. That is the standard your provider uses too.
- "Due date" is not "delivery date." Full term runs from 39 weeks 0 days to 40 weeks 6 days, early term starts at 37 weeks, and late term at 41 weeks. The key-dates list shows each one.
- Private by design. Your dates never leave your device; today's week count is worked out in your browser.
Frequently asked questions
How is a due date calculated from the last period?
Add 280 days to the first day of your last menstrual period. If your cycle is longer than 28 days, add the extra days; if it is shorter, subtract them.
How do I calculate my due date from conception?
Add 266 days to the conception date. Doctors assume pregnancy starts two weeks after the first day of the last period, so this gives the same date as the 280-day rule.
How do you calculate an IVF due date?
Use the transfer date and embryo age: add 261 days for a day-5 (blastocyst) transfer or 263 days for a day-3 transfer, as ACOG recommends.
How many weeks pregnant am I?
Count the days from the first day of your last period to today and divide by 7. The calculator shows weeks and days based on your device's date.
When does each trimester start?
The first trimester runs to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days to the end of pregnancy.
Can my due date change?
Yes. If an early ultrasound measurement differs from your period-based date, your provider may move the due date to match the scan.
Sources
- ACOG Committee Opinion: Methods for Estimating the Due Date (280-day rule, IVF transfers, ultrasound dating) (Last reviewed: October 9, 2026)
- ACOG FAQ: How Your Fetus Grows During Pregnancy (start of pregnancy, trimesters) (Last reviewed: October 9, 2026)
- ACOG Committee Opinion: Definition of Term Pregnancy (Last reviewed: October 9, 2026)
- Stanford Medicine Children's Health: Ovulation Date Calculator (cycle length and ovulation day) (Last reviewed: October 9, 2026)