About Pregnancy Calculator
A pregnancy due date is commonly estimated with Naegele's rule: due date = first day of the last menstrual period + 280 days (40 weeks). This calculator adjusts that for cycle length by adding (cycle length − 28) days. Gestational age is the time from the last period to today, and trimesters change at 13 and 27 weeks. Only a small share of babies arrive on the due date. General estimate, not medical advice.
How to use Pregnancy Calculator
- Fill in "First day of last period" and "Cycle length".
- The result appears below and updates as you type.
- Copy the result with the copy button, or download it as a file where that is offered.
Questions about Pregnancy Calculator
How accurate is a due date based on the last period?
It is an estimate. It assumes ovulation about 14 days before the next period, which varies. An early ultrasound usually dates a pregnancy more precisely, and your doctor or midwife may revise the date.
Why is pregnancy counted from the last period and not from conception?
The date of conception is rarely known exactly, while the first day of the last period usually is. Gestational age therefore starts about two weeks before conception actually takes place.
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