Pregnancy Due Date Calculator
Naegele's rule: add 280 days (40 weeks) to the first day of your last period. This calculator also shows how far along you are today.
How it works
Naegele's rule: due date = LMP + 280 days (equivalently: LMP + 1 year − 3 months + 7 days). Gestational age = days since LMP; week number = completed weeks + 1. Trimesters: weeks 1–13, 14–27, 28–birth.
Example: last period starting 1 January 2026 gives a due date of 8 October 2026. A period starting 15 March 2026 gives 20 December 2026. On 1 April 2026, the January pregnancy is 12 weeks, 6 days — first trimester.
Frequently asked questions
How accurate is the due date?
It is an estimate: only about 1 in 20 babies arrives on the exact date. Most arrive within two weeks either side.
What if my cycles are irregular?
Then Naegele's rule is shaky — an early ultrasound (before 14 weeks) gives a more reliable date.
Why 280 days?
Because pregnancy is counted from the last period, about two weeks before conception. From conception it is roughly 266 days.
What does 'week 13' mean?
You have completed 12 full weeks and are in your 13th week. Trimesters follow the week number: 1–13 first, 14–27 second, 28+ third.