How Do You Calculate the Conception Day of Your Baby?

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Calculated conception dates are always an estimate, the American Pregnancy Association explains, and the presumed date of conception is generally between 11 and 21 days after the first day of the woman's last menstrual period. Women conceiving via artificial insemination can assume that the procedure date is the conception date.

Knowing the exact date of unprotected sexual activity during the possible time span for conception can narrow down the date but cannot ensure complete accuracy down to a single day. Sperm can live in the woman's reproductive tract for up to three days, according to Sexual activity on a particular day may thus result in an actual conception three days later.

Because of the difficulty establishing an exact date of conception, doctors typically use the date of the mother's last menstrual period to determine the gestational age of the baby and estimate a due date, explains the American Pregnancy Association. Doctors generally estimate the due date to be exactly 40 weeks after the first day of the woman's last period, and the fact that only 5 percent of babies are actually born on their due date illustrates the inaccuracy of this guess. Between the eighth and 18th weeks of pregnancy, doctors can use ultrasound technology to estimate the precise age of the fetus.