Reverse Due Date Calculator
When did conception happen? Work backward from an estimated due date or a baby's actual birth date to discover the likely conception date and the timeline of your pregnancy.
Last updated: March 3, 2026
When Intercourse Happened
Because sperm can survive for up to 5 days, the act that led to pregnancy likely occurred in the days leading right up to the estimated conception date.
Saturday, December 13th, 2025
Estimated Timeline
Disclaimer: This tool uses the standard 280-day physiological gestation rule. Because pregnancy lengths naturally vary and ovulation exact timing shifts, these dates are clinical estimates, not guarantees.
How Reverse Conception Math Works
Medical professionals use standard physiological milestones to track pregnancy. Here is how we run that math in reverse based on a 28-day cycle.
Conception Date
Fetal development takes approximately 266 days (38 weeks). Therefore, subtracting exactly 266 days from an EDD finds the day of ovulation and conception.
LMP Date
Clinically, pregnancy is measured from the Last Menstrual Period. Because ovulation happens around Day 14 of a 28-day cycle, the LMP is simply 14 days before conception (280 days total).
Trimesters
The first trimester ends at exactly 13 weeks (91 days after LMP). The second trimester ends upon completing week 27 (189 days after LMP).
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