Conception Date Calculator

Working backward from your due date, this tool estimates when conception most likely occurred and also shows the first day of your last menstrual period (LMP) and a likely fertile window. That date anchors a lot of practical planning: 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.

The estimate rests on the obstetric convention that a due date falls 280 days (40 weeks) after the first day of the LMP, which assumes a 28-day cycle with ovulation on day 14 (ACOG Committee Opinion 700). The calculator therefore subtracts 266 days (38 weeks) from your due date for the conception estimate and 280 days for the LMP, and marks a fertile window from five days before to one day after the conception estimate. The result is still an estimate, because ovulation timing differs from person to person and from cycle to cycle.

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 need not fall on the day of intercourse: in a study of 221 women trying to conceive, pregnancies came from intercourse during the six days ending on the day of ovulation (Wilcox et al., 1995). For the most reliable dating, an early-pregnancy ultrasound reviewed with your healthcare provider remains the gold standard.