Ultrasound Accuracy for Predicting Due Dates

Every pregnant woman wants to know her due date. But a due date calculated from last menstrual period with a due date calculator often doesn't match the due date that is estimated by ultrasound (also known as a sonogram).

During an ultrasound, a technician spreads a warm gel over the lower part of the abdomen, and then presses a tool called a transducer against the belly to examine the fetus using sound waves. An image of the fetus appears on an accompanying computer screen. While looking at this image, the technician takes some standard measurements from different angles and listens for a heartbeat.

While you may be more familiar with this trans-abdominal ultrasound, it is common for doctors to use a transvaginal ultrasound for pregnancy dating, especially early in the first trimester. Sonograms are risk-free, and seeing your baby will likely be an enjoyable experience.

Due Date Accuracy

Naturally, one of the most common questions asked about ultrasound is: Just how accurate are due dates predicted by ultrasound? Evidence suggests that, in the first 20 weeks of pregnancy, the first ultrasound may be the most accurate tool for calculating a fetus's gestational ages.

Due dates predicted with early ultrasound have a margin of error of roughly 1.2 weeks. Doctors will usually keep the original due date (the one generated by the date of the last menstrual period) if the ultrasound due date is within that margin of error.

If you can't remember the date of your last menstrual period date, an early ultrasound can give you a fairly reliable due date. Ultrasound dating is also helpful if your menstrual cycles are irregular.

When Due Dates Don't Match

If an early ultrasound gives you a due date that is 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 that the subsequent ultrasound shows the expected fetal growth for the time period between the scans, your doctor may revise your due date to match the first ultrasound's predictions.

Ultrasounds in Later Pregnancy

It is common for each ultrasound throughout pregnancy to predict a different due date. Earlier ultrasounds are more accurate in terms of predicting the due date, so that's why doctors will usually use the dates and measurements from the first ultrasound of the pregnancy as a reference.

As pregnancy progresses, the accuracy of an ultrasound for predicting due dates decreases. Between 18 and 28 weeks of 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 in pregnancy, ultrasound measurements are more valuable for evaluating the baby's growth over time (compared with earlier measurements) than they are for predicting a due date.