Conception Date Calculator

Working backward from either your due date or the first day of your last menstrual period (LMP), this tool estimates when conception most likely occurred. That window anchors a lot of practical planning in 2026: prenatal appointments, the developmental milestones ahead, and a clearer sense of how far along your pregnancy actually is. What you get is a well-grounded approximation rather than a fixed point on the calendar.

Two standard obstetric approaches drive the estimate. Starting from the LMP, the calculator adds roughly 14 days, on the assumption of a typical 28-day cycle with ovulation near day 14. Starting from the due date instead, it subtracts 266 days (38 weeks), since pregnancy runs about 40 weeks from the LMP and conception lands roughly two weeks after it. Either path lands on an estimate, because ovulation and implantation timing differ from person to person.

Cycle-length irregularities, early or late ovulation, and ordinary biological variation all push the true date away from the average. It also helps to know that conception rarely coincides with the day of intercourse, since sperm can survive several days inside the body. For the most reliable dating, an early-pregnancy ultrasound reviewed with your healthcare provider remains the gold standard.