Naegele's Rule

This due date calculator estimates your due date by adding 280 days to the first day of your last menstrual period — the standard Naegele's Rule used by most clinicians. For cycles other than 28 days, the estimate shifts by the difference, and if you know your conception date, adding 266 days is the most accurate estimate.

Trimester Milestones

The first trimester (weeks 1–13) covers implantation and organ development, the second (weeks 14–26) rapid growth and movement, and the third (weeks 27–40) lung and brain maturation. Your care provider will confirm the estimate with an early ultrasound, which is most accurate between 8 and 13 weeks.

What to Expect

Less than 1 in 20 babies arrives on the exact estimated date. A normal delivery window runs from 37 to 42 weeks, so plan around the date but expect flexibility. Discuss any concerns about gestational age or growth with your healthcare provider.