First, there’s the question of when the pregnancy began: some women know the date of ovulation or conception, but typically, due dates are based upon the woman’s last menstrual period, a method which carries much potential for error.
Second, there’s a question of how long the pregnancy will last.
For related information, see Medscape's Pregnancy Resource Center.
Some parts of popular culture, like baby betting pools, remind us that the date is difficult to guess; however, other messages imply that we can expect a certain date. there is a token message there saying something like “baby’s not here yet?
Well, take care of yourself and be patient.” But clearly the message is to expect the baby then, despite the fact that only 4-5% of babies are born on their exact due date.
The estimation of pregnancy dates is important for the mother, who wants to know when to expect the birth of her baby, and for her health care providers, so they may choose the times at which to perform various screening tests and assessments,such as serum screening, assessment of maturity, and induction of labor for postdate pregnancies.
The 3 basic methods used to help estimate gestational age (GA) are menstrual history, clinical examination, and ultrasonography.