Ultrasounds

Maybe someone else will know. Doesn't sound like worth having if it could, does it?

Depends what you mean by "early". 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 eighteen weeks of pregnancy. Measurement of the sac at five to seven weeks is not accurate. 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. After this the accuracy declines. Between fourteen and twenty-six weeks, measurements of the biparietal diameter, the femur length and abdominal circumference are generally used. The accuracy is plus or minus seven to ten days. The accuracy further declines with advancing gestation age.

So if the pregnancy is between seven and fourteen weeks and the measurement done properly the accuracy is likely to be around 3 days plus or minus. Not four weeks. Even after fourteen weeks the accuracy is still within seven to 10 days.

One wonders why you want there to be a possibility of a four week error?