Question 1

How is the due date calculated from my last period?

Accepted Answer

This tool uses Naegele's rule, a widely used clinical convention: it adds 280 days (40 weeks) to the first day of your last menstrual period, then adjusts for cycles longer or shorter than the standard 28-day cycle.

Question 2

Why would a due date from conception differ from a due date using LMP?

Accepted Answer

A pregnancy is conventionally dated from the first day of the last period, about two weeks before ovulation and conception actually happen. If you enter a known conception date instead, this calculator adds 266 days rather than 280 to reach the same approximate 40-week mark.

Question 3

How accurate are due date calculations like this?

Accepted Answer

These formulas assume a textbook-regular cycle and known ovulation timing, so they give a reasonable estimate but not a guarantee — most healthy pregnancies deliver anywhere in a multi-week window around the calculated date. A first-trimester ultrasound is generally considered the most accurate way to confirm gestational age.

Question 4

What are trimesters based on?

Accepted Answer

Trimesters are commonly defined as weeks 1–12 (first), weeks 13–26 (second), and week 27 onward (third), though the exact week boundaries can vary slightly between sources and healthcare providers.

Pregnancy Due Date Calculator

How the due date is calculated

Common questions