Knowing the exact day of conception is a difficult thing. Sperm can live in a woman’s body for up to five days, making it impossible to calculate exactly when conception occurred. It is not a fine science.
In addition, women ovulate anywhere from 11 to 21 days after the first day of their last period. Ovulation varies from person to person. This is the reason why due dates are calculated from the 1st day of the last period and not from the date of conception.
Even though this can seem confusing, it has become a standard way of measuring pregnancy and calculating a due date. The due date is normally set at 40 weeks after the first day of the woman’s last period.
Fortunately, an ultrasound can help to determine gestational age. Ultrasounds are usually most accurate between the 8th and the 18th weeks. With an ultrasound, even women who don’t have periods at all can tell how far along they are.
Since only about 5% of all babies are actually born on the due date, this is only an estimation to begin with. Each baby and woman are unique individuals with their own timetables for birth. All we can do is give them a general idea what to expect!
Assure Women’s Center: If You Think You Might be Pregnant, We’re Your Best First Step!