How your due date is calculated is often confusing and different from what you expect. Pregnancy is 40 weeks or 280 days long, starting from the first day of the last menstrual period. This means that at the time of conception, a woman is already two weeks pregnant. This assumes that her menstrual cycles are 28 days apart.

Many times women do not remember the exact day of their last menstrual period, or they may not have menstrual cycles that are 28 days long, or they might’ve been on birth control at the time of conception. Sometimes women just ovulate a little bit late for the bleeding that they thought was there last menstrual period was actually something different, like implantation bleeding. All of these factors contribute to dating based on the last menstrual period being wrong about 40% of the time. Your doctor will compare the date derived from the last menstrual period dating to a date determined by an ultrasound, and in some cases your due date will be changed based on the ultrasound.

The first ultrasound performed that is able to give a measurement for the baby is always the most accurate for determining the due date. Sometimes, patients are confused because subsequent ultrasounds will show a baby that has a slightly different due date and they will wonder if they’re due date should be changed again based on these later ultrasound. The answer is no. The later ultrasound reflect a baby that is just a little bigger or smaller than the average for that gestational age or very just because of the margin of error.

Make sure you clarify at one of your first prenatal visit what your final estimated due date is; then, don’t get too fixed on that day. Only 2% of women deliver on their due date. Maybe we should’ve called it a due month.