How to Calculate Your Pregnancy Due Date
A due date is built from a 280-day rule that assumes a 28-day cycle. If your cycle is not 28 days, the standard answer is wrong — and correcting it is straightforward.
- Naegele's rule: last period plus 280 days. That is the number nearly every due date starts from.
- The rule assumes a 28-day cycle with ovulation on day 14. A 35-day cycle pushes the true date about a week later.