How Is My Due Date Calculated? The Complete Clinical Guide

When you first discover you are pregnant, the immediate question is: “When is my baby due?” In clinical medicine, your estimated date of delivery (EDD) is not a prediction of the exact day labor will begin. Rather, it represents the specific calendar day that marks 40 completed weeks of gestational age.

1. The Classic Rule: Naegele's Rule

Developed by 19th-century German obstetrician Franz Karl Naegele, this remains the global foundation of clinical pregnancy dating. The standard formula operates as follows:

2. Adjusting for Menstrual Cycle Length

Naegele's original rule assumes an exact 28-day cycle with ovulation occurring precisely on day 14. However, research demonstrates that normal follicular phases vary between 10 and 25 days.

• For 35-day cycles: Ovulation occurred around day 21 (7 days later than average). Your due date is LMP + 287 days.
• For 24-day cycles: Ovulation occurred around day 10 (4 days earlier). Your due date is LMP + 276 days.

3. First-Trimester Ultrasound Dating (The Gold Standard)

According to the American College of Obstetricians and Gynecologists (ACOG Committee Opinion No. 700), early ultrasound measurement of the fetal Crown-Rump Length (CRL) before 14 weeks is the single most accurate method for establishing gestational age:

• Under 9 weeks: If ultrasound differs from LMP by more than 5 days, the ultrasound date is used.
• 9 to 13 6/7 weeks: If ultrasound differs by more than 7 days, the ultrasound date replaces the LMP estimate.