How to Calculate Your Due Date
Your estimated due date (EDD) is roughly 40 weeks from the first day of your last menstrual period. Here are the three most-used methods.
Method 1: LMP (Naegele's Rule)
Take the first day of your last menstrual period, add 1 year, subtract 3 months, then add 7 days. Example: LMP March 10 → EDD December 17 of the following year.
This is the standard method used by clinicians and assumes a 28-day cycle with ovulation on day 14.
Method 2: Conception Date
Add 266 days (38 weeks) to the date of conception. Useful if you tracked ovulation or used assisted reproduction.
Method 3: Ovulation Date
Same math as conception (266 days from ovulation). Most accurate when you've tracked LH surges, BBT, or had timed ultrasounds.
When the methods disagree
An 8–12 week dating ultrasound is the gold standard. If LMP and ultrasound differ by more than 7 days in the first trimester, ultrasound usually wins.