Pregnancy Due Date
Estimated delivery date from the last menstrual period.
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The formula
due date = LMP + 280 days (Naegele's rule)
Naegele's rule adds 280 days — 40 weeks — to the first day of the last menstrual period. It assumes a regular 28-day cycle with ovulation at day 14, so it is an estimate: only about 4% of babies arrive on the exact date, and most births fall within two weeks either side.
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Frequently asked questions
How is the due date estimated?+
Naegele's rule adds 280 days (40 weeks) to the first day of your last menstrual period. Only about 4% of babies arrive exactly on the due date.
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