How to Calculate Your Conception Date from Gestational Age

Conception date is estimated by subtracting the gestational age from the current date (or ultrasound date). Gestational age is measured from the first day of the last menstrual period (LMP), which precedes conception by ~2 weeks in a typical 28-day cycle.

Example

Given: September 9, 2009 at 5 weeks 6 days gestation.

1. 5 weeks 6 days = 41 days.
2. Count backward 41 days from Sep 9, 2009 → August 31, 2009 (LMP).
3. Conception ≈ LMP + 14 days = September 14, 2009.

Note: The original text incorrectly stated the conception date as August 31. That date is the estimated LMP, not conception.

Clinical Practice

• First-trimester ultrasound (crown-rump length) is the most accurate dating method (±5–7 days).
• LMP dating assumes regular 28-day cycles; adjust for cycle length.