How due dates are estimated
Naegele's rule, ultrasound dating, and conception-based math — how a due date actually gets set.
4 min readReviewed May 1, 2026
Quick answer
The standard estimate is 280 days from the first day of your last period. Early ultrasounds often refine this number and become the working due date.
The three common methods
1. Naegele's rule (LMP)
First day of last period + 1 year − 3 months + 7 days.
2. Conception date
Add 266 days. Useful when ovulation/conception is known.
3. Ultrasound dating
Measurements from a first-trimester scan; the most accurate of the three.