Pregnancy due date and gestational age calculator

Pregnancy is dated from the first day of the last menstrual period (LMP), 280 days (40 weeks) before the due date. Naegele’s rule gives about the same date on the calendar: add one year, subtract three months and add seven days. Conception is taken as 14 days after the LMP, and ACOG dates IVF pregnancies from the embryo’s age at transfer.

From LMP:          due date = LMP + 280 days (+ cycle length − 28)
From conception:   due date = conception + 266 days
From IVF transfer: due date = transfer + 266 − embryo age (day 5: + 261; day 3: + 263)
Gestational age = date − (due date − 280 days)

Trimesters here: first up to 13 weeks 6 days, second 14 weeks 0 days to 27 weeks 6 days, third from 28 weeks. ACOG’s term definitions: early term 37–38 weeks, full term 39–40 weeks, late term 41 weeks, post-term from 42 weeks.

Assumptions and limits

• Assumes a regular cycle with ovulation about 14 days before the next period; an early ultrasound measurement can change the dating.
• Dates are calendar days with no time zone; “today” is your device’s date.

Worked example

An LMP of 2026-03-01 gives a due date of 2026-12-06 (280 days later); Naegele’s calendar rule gives 2026-12-08, because the nine months from 8 March include more 31-day months. A day-5 embryo transferred on 2026-03-19 gives 2026-12-05 (261 days later).

References

• ACOG Committee Opinion 700: methods for estimating the due date
• ACOG Committee Opinion 579: definition of term pregnancy