Conception Date Calculator: Determining Conception from Due Date

Standard formula: Conception date = Due date – 266 days (average fetal gestational length).

Example

Due date: April 24, 2011
Conception: April 24, 2011 – 266 days = July 31, 2010

Context

• 266 days = 38 weeks = fetal age (conception to birth)
• 280 days = 40 weeks = gestational age (LMP to birth)
• 14-day difference = average LMP-to-ovulation interval

Limitations

• Assumes regular 28-day cycle, ovulation day 14
• Normal variation: ovulation day 10–20+
• Sperm survival up to 5 days
• Clinical dating uses early ultrasound (CRL), not this math

Approximation only. Actual conception window spans several days.