Calculates the date 280 days after the last menstrual period.
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Calculation basis and official references
Reviewed on:
This calculator is not a diagnosis, treatment plan, or prescription. Please consult a qualified clinician about your medical decisions.
Formula basis
Due date = last menstrual period + 280 days.
Calculation method
Calculates the due date as 280 days (40 weeks 0 days) after the first day of the last menstrual period.
Exclusions and checks required
Results can differ from the actual due date when the menstrual cycle is not 28 days or ovulation timing varies (see CQ009 of the JSOG/JAOG obstetric practice guideline).
Early-pregnancy ultrasound dating (e.g. crown-rump length) should be prioritized clinically; this calculation is a simple reference based only on the last menstrual period.
Example input:：Last menstrual period January 1, 2026
lmp
2026-01-01
October 8, 2026 is a theoretical due date based only on the last menstrual period and can differ from the actual delivery date.