Pregnancy & Due Date Calculator

Naegele's rule: due date = first day of the last period + 280 days. An LMP of 1 January 2026 gives roughly 8 October 2026. This calculator also adjusts for your cycle: a 35-day cycle gives LMP + 287 days; a 21-day cycle gives LMP + 273. That adjustment matters because the base rule assumes 28 days.

From the LMP, not from conception. So in weeks one and two you technically aren't pregnant yet — conception usually happens around week two or three. The convention exists because the last period is far easier to recall than the date of conception. The calculator shows weeks plus days, and your current trimester.

First trimester is weeks 1–12, second is 13–27, third is 28 to birth. The calculator shows your current trimester and estimated start dates for the next. This split is the common convention; some sources place the second trimester at week 14.

For regular cycles it's a reasonable first estimate. For irregular cycles accuracy drops, because the rule leans on a day-14 ovulation assumption that may not hold. A first-trimester ultrasound, especially before 14 weeks, dates a pregnancy far more reliably by measuring the fetus directly. If a scan disagrees with this figure, follow the scan and your doctor.

Because it's a statistical estimate, not a schedule. Term birth spans weeks around it, and natural pregnancy length genuinely varies between people. First pregnancies, previous history, and ordinary biological variation all play a part. Read the due date as the middle of a window rather than an appointment.