Calculate your estimated due date (EDD) from your last menstrual period or conception date.
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How This Works
Naegele's rule (LMP method): EDD = LMP + 280 days (40 weeks), adjusted for cycle length difference from 28 days. Conception method: EDD = conception date + 266 days (38 weeks). IVF 5-day blastocyst: EDD = transfer date + 263 days. The dating scan ultrasound at 12 weeks is considered more accurate than the LMP method for establishing gestational age.
The EDD is an estimate based on average pregnancy length of 40 weeks from LMP. Individual variation is large: normal delivery ranges from 37–42 weeks. Dating scan at 10–13 weeks is the most accurate method (±5–7 days). LMP-based calculation assumes regular 28-day cycles, which many women do not have. If your scan date differs from LMP calculation by more than 7 days, the scan date is used.