Every pregnant woman wants to know her due date, and due dates calculated from a last menstrual period often don't match the due date estimated by the first ultrasound. So naturally one of the most common questions about ultrasound accuracy is: How accurate are the due dates from an ultrasound?
Evidence suggests that, in the first 20 weeks of pregnancy, due dates generated by the first ultrasound may the most accurate tool for predicting your baby's birth. But ultrasound due dates have a margin of error of roughly 1.2 weeks, so doctors will usually keep the original due date (from your last menstrual period date) if the ultrasound due date is within that margin of error.
If you can't remember your last menstrual period date or have consistently irregular cycles, the doctor can use an early ultrasound to give you a fairly reliable due date.
When Dates Don't Match
If your first ultrasound returns a due date more than 1.2 weeks away from what was expected, there is a good chance that everything is still fine. You may have simply conceived earlier or later than you thought you did (which can happen if your cycle is at all irregular or if you remembered your last menstrual period date incorrectly).
Your doctor might want to repeat the ultrasound to make sure that your pregnancy is developing as it should. Assuming the subsequent ultrasound shows the expected growth for the time period between the scans, your doctor may revise your due date to match the first ultrasound's predictions. And as long as the repeat ultrasound shows consistent growth and there are no signs of problems, there is no reason to be concerned if your doctor changes your due date.
It is common that each ultrasound throughout the pregnancy will return a different due date. Earlier ultrasounds are more accurate for predicting the due date, so doctors will usually use the dates and measurements from the first ultrasound of the pregnancy as a reference.
As the pregnancy progresses, the accuracy of ultrasound for predicting due dates decreases. Between 18 and 28 weeks' gestation, the margin of error increases to plus or minus two weeks. After 28 weeks, the ultrasound may be off by three weeks or more in predicting a due date. Thus, later pregnancy, ultrasound measurements are more valuable for evaluating the baby's growth over time (compared to earlier measurements) than they are for predicting a due date.
Accuracy of Ultrasound Dating. University of Florida. Accessed: Jan. 20, 2009.
Barnhart KT, Simhan H, Kamelle SA. "Diagnostic accuracy of ultrasound above and below the beta-hCG discriminatory zone." Obstetrics & Gynecology 1999. 94(4):583-7.
Mongelli, Max MB BS; Wilcox, Mark MD; Gardosi, Jason MD. "Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates." American Journal of Obstetrics and Gynecology 1996. 174(1 Pt 1):278-81.