How to Calculate Your Pregnancy Due Date

A pregnancy due date—technically the Estimated Due Date (EDD)—is calculated as approximately 40 weeks (280 days) from the first day of your last menstrual period (LMP). Most pregnancies deliver within 2 weeks of this date.

Method 1: Naegele's Rule (Most Common)

Named after the 19th-century physician Franz Naegele:

EDD = LMP + 1 year − 3 months + 7 days

Example: LMP = January 15

  1. Add 1 year: January 15 of next year
  2. Subtract 3 months: October 15
  3. Add 7 days: October 22

Method 2: LMP + 280 Days

Simply add 280 days to the first day of your last period.

LMP = January 15 → + 280 days = October 22 ✓

Method 3: Ovulation/Conception Date

If you know your conception date:

EDD = Conception date + 266 days (38 weeks)

Gestational Age vs. Fetal Age

  • Gestational age (used in medicine): Counted from LMP. At 6 weeks pregnant, the embryo is actually ~4 weeks old.
  • Fetal/Embryonic age: Counted from conception (ovulation), about 2 weeks less than gestational age.

Ultrasound Dating

Early ultrasound (before 13 weeks) is the most accurate way to confirm or adjust your due date. After 20 weeks, ultrasound dating becomes less reliable due to natural size variation between fetuses.

How Many Babies Arrive "On Time"

Only about 4% of babies are born on their exact due date. Roughly 80% are born within 2 weeks before or after. Pregnancies between 39–40 weeks are considered "full term."

Use our due date calculator to find your EDD from any LMP date.