Due Date AccuracyA woman's due date is best determined if the woman knows the date of ovulation, and can better guess the date of conception. Most women, however, are unaware of this and must base due date calculations on their last menstrual period. No matter which it is based on, due dates are generally an estimate with room for error. Due dates are calculated using Naegle's Rule, a formula first developed in 1812.. The formula for Naegle's Rule is (LMP+7 days)-3 days=due date; LMP being last menstrual period. Using Naegles Rule, the average gestation is 260 days from the last menstrual period, or 288 days total. Two-thirds of all babies are born within ten days of their predicted due date; eighty percent of babies are born thirty-eight to forty-two weeks after the last menstrual period. There are, however, many variations. Caucasian mothers with no children should add fifteen days to their due date; Caucasian mothers who have children should add ten. African-American mothers generally have shorter gestations, as do Asian women; they should add eight and a half days. Other factors include the quality of prenatal care and other factors that can only be determined by doctors. When predicting a due date, the fetal heart rate should be considered as should the lung development. Doctors should also consider the first time the mother feels movement, and uterus size when measured during pregnancy. In general, once pregnant, an expectant mother should assume her baby will be born anywhere from two weeks before to two weeks after the predicted due date. Read more of our articles on the pregnancy due date Calculating Your Pregnancy Due Date |