How it works
We use Naegele's rule: the due date is 280 days (40 weeks) after the first day of the last menstrual period, for a 28-day cycle, adjusted if your cycle is longer or shorter. From a conception date, we add 266 days.
Example
Last period on January 1, 2026, with a 28-day cycle: estimated due date October 8, 2026.
Only about 5% of babies arrive on their due date. A first-trimester ultrasound usually gives the most accurate estimate.
Frequently asked questions
What is gestational age?
The length of the pregnancy counted from the first day of the last period, in weeks and days.
See also
Results are estimates for information only and are not financial, legal or medical advice. Rates, taxes and fees vary — confirm with a professional or your lender.