Estimate a due date from the first day of the last menstrual period, adjusted for your usual cycle length. This is a calendar estimate, not clinical dating.
Estimated due date = first day of last period + 280 days + (usual cycle length − 28 days).
For a last period starting January 1, 2026 and a 28-day cycle, the estimated due date is October 8, 2026. A 30-day cycle moves this arithmetic estimate two days later.
This assumes ovulation changes with cycle length in a simple way. Irregular cycles, assisted conception and uncertain dates need different interpretation. Ultrasound dating and clinical assessment may revise the estimate. A due date is not a prediction of the exact birth day.
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Editorial responsibility: Nicolas Belotti · Methods and testing · Report an error
English edition: 16 September 2026.