By CalculatorInn Team · Updated: 2026-03-22 · Free & accurate · Instant results
Estimate your baby's due date based on the first day of your last menstrual period (LMP) using Naegele's rule.
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Due Date = LMP + 280 days (40 weeks)
Naegele's rule adds 280 days (40 weeks) to the first day of the last menstrual period to estimate the due date.
Only about 5% of babies are born on their exact due date. Most arrive within 2 weeks before or after.
A standard method: add 1 year, subtract 3 months, and add 7 days to your LMP date. It equals adding 280 days.
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