Calculate due date

Estimate a pregnancy due date and gestational milestones from the first day of the last menstrual period.

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ToolDue Date Calculator
Input
Output
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How it works

The conventional 28-day calculation adds 280 days to the menstrual date, while a longer or shorter cycle shifts due date and estimated ovulation by its difference from 28. Gestational age still starts at the menstrual date, and the two trimester markers fall at 14 weeks and 28 weeks.

Formula
estimated due date = first day of last menstrual period + 280 days + (cycle length − 28 days)
  • Twenty-eight days is the default because the conventional rule assumes ovulation on day 14 of a regular 28-day cycle.
  • The result is a menstrual estimate and does not revise a clinician's ultrasound-based best obstetric estimate.

Worked example

A 28 day cycle
Last period started on 1 March 2026
Input
											2026-03-01
Output
												Due date: 2026-12-06
Conception approx: 2026-03-15
Second trimester: 2026-06-07
Third trimester: 2026-09-13
Weeks along: 25 weeks and 2 days
Days to go: 103

When to use this

Prenatal intake records compare last-menstrual-period dating with early ultrasound, pregnancy calendars mark 14- and 28-week boundaries, and obstetric records express gestational age in completed weeks and days.

Edge cases

  • An impossible value such as 2026-02-30 is rejected rather than being normalised into a March date.
  • A menstrual date in the future is rejected before it can produce negative weeks of gestation.
  • Irregular cycles, uncertain recall, assisted reproduction, and a discrepant early ultrasound are not reconciled; the entered menstrual date remains the sole basis.

References