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.

Is due date based on LMP vs ultrasound?

LMP versus early ultrasound If the ultrasound date is within seven days of your LMP date, we would stick with your LMP date. Ultrasounds done later in the pregnancy are less accurate for dating, so if your due date is set in the first trimester, it shouldnt be changed.

How far along do you have to be to have an ultrasound done?

Most women get an ultrasound in their second trimester at 18 to 20 weeks of pregnancy. Some also get a first-trimester ultrasound (also called an early ultrasound) before 14 weeks of pregnancy. The number of ultrasounds and timing may be different for women with certain health conditions like as asthma and obesity.