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Due date calculator
Estimate your baby’s due date from the first day of your last menstrual period, and see how many weeks pregnant you are today and which trimester you are in.
How to use it
Enter the first day of your last period and your usual cycle length (28 days by default). Your midwife or doctor will confirm the date with the first ultrasound.
Formula and method
Naegele’s rule: due date = first day of last period + 280 days (40 weeks), adjusted by (cycle length − 28) days. Pregnancy weeks are counted from the last period.
Worked example
Last period on 1 March 2026 with a 28-day cycle: due date = 1 March + 280 days = 6 December 2026; conception around 15 March.
Frequently asked questions
How accurate is the due date?
Only about 4% of babies are born on their due date; most arrive within two weeks before or after it. The first-trimester ultrasound gives the most reliable date.
When does each trimester start?
The 1st trimester runs to the end of week 13, the 2nd from week 14 to week 27 and the 3rd from week 28 until birth.