Every pregnant woman wants to know her due date, and a due date that is calculated from her last menstrual period often doesn't match the due date that is estimated by her first ultrasound (also known as a sonogram).
If your first 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.Naturally, one of the most common questions asked about ultrasound accuracy is: Just how accurate are the due dates that are predicted by an ultrasound?
Evidence suggests that, in the first 20 weeks of pregnancy, the first ultrasound may be the most accurate tool for predicting when your baby will be born.
But early ultrasound due dates have a margin of error of roughly 1.2 weeks, so doctors will usually keep the original due date (the one generated by the date of your last menstrual period) if the ultrasound due date is within that margin of error.
If you can't remember your last menstrual period date or if you have consistently irregular cycles, the doctor can use an early ultrasound to give you a fairly reliable due date.
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 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.