How the due date is estimated
The most common method is Naegele's rule: pregnancy is counted as about 280 days (40 weeks) from the first day of your last menstrual period (LMP). This calculator uses that, and adjusts for cycle length if yours differs from the typical 28 days, since ovulation timing shifts the date slightly.
Why it's an estimate
Only about one in twenty babies arrives exactly on the predicted date. A normal, full-term birth can happen anywhere from three weeks before to two weeks after. The LMP method also assumes regular cycles and known dates; an early ultrasound is more accurate and is what clinicians rely on. Use this number as a helpful guide for planning, not a fixed deadline.
Good to know
- Full term is 37-42 weeks.
- An ultrasound dating scan is more precise than LMP.
- Irregular cycles make the estimate less reliable.
- Always confirm with your healthcare provider.
Frequently asked questions
Is the due date exact?
No. It's an estimate; most births occur within a couple of weeks either side.
What if my cycle isn't 28 days?
Enter your average length and the calculator adjusts the date.
Is my data private?
Yes — everything is calculated in your browser.