Calculator
Conception Date Calculator
Work backward from a due date or last period to estimate when conception most likely occurred. Useful for dating scans and paternity timelines.
Enter a date above to estimate your conception date.
How conception dates are estimated
Unless you tracked ovulation precisely or had a single instance of intercourse, the exact date of conception is rarely known. Doctors estimate it from two anchors. From your last menstrual period (LMP), ovulation — and therefore conception — is assumed to occur around day 14, so the estimate is LMP + 14 days. From your due date, conception is estimated by subtracting 266 days (38 weeks), giving Due Date − 266 days.
The five days before ovulation plus the day of ovulation make up your most fertile window, because sperm can survive up to five days inside the body while the released egg remains fertilizable for about 24 hours.