How the due date is estimated
The most common 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, so if your cycle is longer or shorter, adjusting the cycle length shifts the estimate accordingly. Your current progress is measured the same way — in completed weeks and days since that first day.
Why it's only an estimate
Only about one in twenty babies arrives exactly on the estimated date; most births happen in the two weeks either side. Cycle variation, the timing of ovulation and later ultrasound measurements can all move the date. An early ultrasound is generally the most accurate way to date a pregnancy.
Trimesters at a glance
Pregnancy is usually divided into three trimesters: roughly weeks 1–12, 13–26 and 27 to birth. This tool shows which one you're in based on completed weeks, alongside the estimated due date, so you have a simple sense of where you are.
Note: this calculator is for general information only and is not medical advice. Always confirm dates and any concerns with a qualified healthcare provider.
Frequently asked questions
What if I don't know my last period date?
Then this estimate won't be reliable. An early dating ultrasound from your provider is the best way to estimate the due date when the last period date is uncertain.
How accurate is the due date?
It's a guide, not a guarantee. Most births occur within two weeks before or after the estimated date. Treat it as approximate and confirm with your provider.