Your due date is one of those highly-anticipated pieces of pregnancy information – right up there with your baby’s sex. Once your doctor confirms your pregnancy, he or she will tell you your estimated due date (EDD).
The most common method used to calculate a woman’s estimated due date is to count from the date of her last menstrual period. However, for women with irregular menstrual cycles, those who were recently on hormonal birth control, or who have first trimester bleeding, this can lead to an inaccurate due date.
An early ultrasound can improve the accuracy of the EDD, but it’s still not an exact science. In fact, the margin of error for an ultrasound can be surprisingly broad and depends on when you have your ultrasound.
For instance, an ultrasound performed between 12 and 20 weeks may be inaccurate by one week; one that is performed between 20 and 30 weeks has a margin of error of two weeks; and one performed after 30 weeks may be three weeks off. So if you have an ultrasound at what is thought to be your 31st week and it shows you to be at 35 weeks, you could actually be anywhere from 32 weeks to 38 weeks! This can cause potentially serious problems if you go past your due date and your doctor induces labor, when in fact your pregnancy is not yet full term.
At your prenatal appointments beginning mid-way through your pregnancy, your doctor will measure your fundal height, or the distance from your pubic bone to the top of your uterus. The measurement in centimeters should roughly correspond to how many weeks pregnant you are – so you should measure approximately 24 centimeters around your 24th week. This will not only help your doctor estimate your baby’s size and growth rate, an unusual measurement may also indicate an incorrect due date.
To determine and confirm an accurate due date, it is important to make an appointment with your doctor as soon as you find out you are pregnant and continue to get regular checkups throughout your pregnancy. The measurements and ultrasounds taken periodically will help narrow the due date margin of error and ensure your baby is growing properly and that it is born at the right time.