Reverse Conception Calculator (by Due Date)
Backtrack from a birth date or due date to find the likely date of intercourse. Features a romantic weekend estimator for fun planning.
About
This tool is often used for curiosity or retrospective planning. By reversing the gestation timeline, we can pinpoint the likely weekend or timeframe when conception occurred. While medical due dates are precise estimates, babies arrive on their own schedule. Only 4% of babies are born on their exact due date. Therefore, the reverse calculation provides a probability window rather than a certainty. It is useful for parents wanting to know "when it happened" or trying to replicate the timing for future planning.
Formulas
We assume a standard gestation of 266 days from conception to birth.
The sperm viability window extends this range backwards by up to 5 days.
Reference Data
| Event | Timeline (Reverse) | Notes |
|---|---|---|
| Birth / Due Date | Day 0 | Target Date |
| Full Term | −2 Weeks | 38 Weeks Gestation |
| Viability | −16 Weeks | 24 Weeks Gestation |
| First Trimester End | −27 Weeks | 13 Weeks Gestation |
| Conception | −38 Weeks | Week 2 (Fetal Age 0) |
| LMP | −40 Weeks | Start of Cycle |