What it does
Two OB tools in one: a GTPAL from a pregnancy history, and an estimated due date from the last menstrual period.
How it works
GTPAL is Gravida (pregnancies), Term, Preterm, Abortions and Living children. The due-date tool adds 280 calendar days (40 weeks) to the LMP, using the regular 28-day-cycle assumption. This avoids calendar-month rollover issues.
Worked example
An LMP of January 1, 2026 gives an estimated due date of October 8, 2026. A twin birth counts as one pregnancy but two living children.
Good to know
Naegele’s rule assumes a regular 28-day cycle and is an estimate.
Open the interactive calculator