"Very few expectant moms know exactly when they conceived, however. Even if you only had sex once during your fertile period, you wouldn't necessarily conceive right then. It's possible that the sperm hung out in your fallopian tube for a few days until your egg came along, at which point it fertilized the egg and your baby was conceived.
So without knowing the day of conception, how does anyone determine a due date?
Most people just estimate. The method typically used by healthcare providers is simply to count from the first day of your last period. For women with a menstrual cycle of average length, that day is usually about two weeks before conception, which explains why pregnancies are said to last 40 weeks. This method doesn't take into account how long your menstrual cycle actually is or when you think you might have conceived."