How accurate is ultrasound, and what are possible sources of error?
The accuracy of ultrasound for dating a pregnancy depends on at what point during the pregnancy the ultrasound is taken. Pregnancy dating is most accurate during the first half of pregnancy. Measurement of the sac at five to seven weeks gives an accuracy of plus or minus ten days. Measuring the crown-rump length gives an accuracy of plus or minus three days at seven weeks; this test can be used from the seventh to the fourteenth week. Between fourteen and twenty-six weeks, the measurement of the biparietal diameter of the baby’s head, the femur length, and the head and abdominal circumference is used; the accuracy is plus or minus seven to ten days. Later in pregnancy, the accuracy declines, and may be plus or minus twenty-one days. —————————————– From Dr. T. Reynolds: The reason for this is that different babies grow at different rates and that all measurements are subject to inacccuracy because the object being measured is not linear (e.g. a babies head is
