How to use this calculator
- Enter or choose first day of last period.
- Enter or choose usual cycle length.
- Enter or choose calculate for date.
- The you are updates as you type. Use Copy result or Share to keep it.
Method
Start = LMP + (cycle length − 28) days; Gestational age = date − start; Due date = start + 280 days
- LMP
- first day of the last menstrual period
- cycle length
- days from one period to the next (28 if unknown)
Worked example
- From 1 March to 15 September 2026 is 198 days.
- 198 days = 28 weeks and 2 days, which is in the third trimester (from 28 weeks).
- Due date = 1 March 2026 + 280 days = 6 December 2026, 82 days after 15 September.
With a last period starting 1 March 2026, on 15 September 2026 you are 28 weeks and 2 days pregnant, with an estimated due date of 6 December 2026.
What the result means
The due date is an estimate: relatively few babies arrive on the day itself, and birth from 37 weeks onward is considered term. If you have had a dating ultrasound, the scan-based dates in your notes take priority over this calculation.
Assumptions and limits
- Pregnancy dated from the LMP by convention: due date = LMP + 280 days (ACOG).
- Longer or shorter cycles shift the dates by (cycle − 28) days, assuming ovulation 14 days before the next period.
- Trimesters: first to 13 weeks 6 days, second 14 weeks to 27 weeks 6 days, third from 28 weeks.
- Irregular cycles, recent hormonal contraception or an uncertain LMP make date-based estimates unreliable.
- A first-trimester ultrasound is the most accurate way to date a pregnancy.
Questions
Why am I “2 weeks pregnant” at conception?
Pregnancy weeks are counted from the first day of your last period, not from conception, so the first two weeks happen before you conceive.
My scan gave a different due date. Which is right?
Use the scan date. ACOG advises changing the due date to the ultrasound estimate when the difference is larger than a set threshold (for example, more than 7 days at 9–13 weeks).
Sources
Please note: This is a general screening estimate for adults, not medical advice or a diagnosis. Talk to a qualified health professional about your health.