The due date is one of the first numbers you get in pregnancy, and one of the most misunderstood. It is a useful planning estimate — but it is an estimate, not an appointment your baby agreed to.
TL;DR — Enter the first day of your last period in the due date calculator to estimate your due date and see how far along you are.
How the date is estimated
The standard method, Naegele’s rule, adds 280 days — 40 weeks — to the first day of your last menstrual period. It assumes a regular 28-day cycle with ovulation around day 14. That is why pregnancy is counted from your last period rather than from conception, which happens about two weeks later.
Why it’s only an estimate
Cycles vary, ovulation can shift, and babies develop on their own schedule. Only about 1 in 20 babies arrives exactly on the due date; most come within a couple of weeks either side. A full-term birth is anywhere from 37 to 42 weeks, so a few days’ difference is entirely normal.
What’s more accurate
If your cycle is irregular or you are unsure of your dates, an early dating ultrasound is more accurate than the calendar method, especially in the first trimester. Use the due date calculator for a quick estimate, and let your clinician’s scan refine it.