Reverse Due Date Calculator
If you already know your due date and want to figure out when conception likely happened, this calculator works backward through the math. It reverses Naegele's rule and adjusts for your cycle length to estimate the conception date and your fertile window.
Standard due date calculators start from the first day of your last menstrual period and add 280 days. This calculator does the opposite. Given a due date, it subtracts 280 days to find the LMP, then moves forward to the estimated ovulation day to pinpoint conception.
Conception typically happens on or near the day of ovulation, which is about 14 days before the start of the next period. For a 28-day cycle, that puts ovulation around day 14. For a 32-day cycle, ovulation shifts to around day 18. The fertile window stretches from about 5 days before ovulation through 1 day after, because sperm can survive 3 to 5 days in the reproductive tract.
Keep in mind that these are estimates. Even with regular cycles, ovulation can shift by a few days. First-trimester ultrasound remains the gold standard for dating a pregnancy. If you need precise dating for legal or medical reasons, consult your healthcare provider.