The conception date calculator works backwards from the estimated due date to estimate when conception occurred. It subtracts 266 days (38 weeks) from the due date, as fertilisation to birth averages 266 days. The last menstrual period is estimated by subtracting 280 days (40 weeks) from the due date.
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Pro Tip
Due dates from ultrasound (especially in the first trimester) are typically more accurate than LMP-based calculations, as cycle length varies and ovulation timing differs between women.
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