How a due date is estimated
The standard method, called Naegele's rule, counts 280 days (40 weeks) from the first day of your last menstrual period. It assumes a 28-day cycle with ovulation on day 14, so this calculator shifts the date by the difference if your cycle is longer or shorter.
Only about 1 in 20 babies arrive on their due date. Most are born within two weeks either side. An early ultrasound gives a more accurate date than the calendar method, especially if your cycles are irregular, and your care provider's date should always take priority.
Worked example
If your last period started on January 10, 2026 and your cycle is 28 days, add 280 days: the estimated due date is October 17, 2026. With a 32-day cycle, ovulation is likely about 4 days later, so the date moves to October 21, 2026.
To estimate when you were fertile, use the ovulation calculator.
Frequently asked questions
How many weeks is a full-term pregnancy?
Pregnancy is counted as 40 weeks from the last period. Babies born from 39 weeks to 40 weeks and 6 days are considered full term.
When does each trimester start?
The second trimester starts at 13 weeks and the third at 28 weeks, counted from the first day of the last period.
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