Enter the first day of your last period and your usual cycle length.
Formula
Due date = first day of last period + 280 days (adjusted for cycle length)
Worked example
A last period on 1 August 2026 with a 28-day cycle gives a due date of 8 May 2027.
Frequently asked questions
How accurate is the due date?
Only about 4% of babies arrive on the due date; most come within two weeks either side.
Does an ultrasound change the date?
Yes. An early ultrasound often gives a more reliable date.
Why count from the last period?
Because it is a date most people know; conception usually happens about two weeks later.
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Results are estimates for information only. Check important figures with a professional.