How accurate are first trimester ultrasounds at 8w1d & 9w5d done 2 weeks apart? and how much can it be off by?

Very accurate.: Hi!, great question! Vaginal ultrasounds performed at those times are typically very good at visualizing a few things: -The location of a pregnancy. -If there is one fetus or more than one. -The presence or absence of a heart beat. -And, how large the fetus is from it's head to it's bottom (otherwise known as the crown-rump length). Usually these early ultrasounds have an error range of 3-7 days.

Accurate: They are accurate depending on technique / skill of the tech and the expertise of the radiologist. But they can off by up 1 week.

Pretty Accurate: +/- about 3-4 days if done properly by skilled/experienced operator. With the dates you list, they would be within the error of measurement and suggest your pregnancy is growing appropriately to date. Hope this helps. Best of luck. Dr R

Fetal Ultrasound: A first trimester ultrasound is pretty accurate in terms of predicting expected date of delivery. Having two within 2 weeks with these fetal ages tends to indicate the accuracy of the measurements, and indicates that the fetus is growing. If the 2 ultrasounds were 14 days apart, the difference in the estimated age was 11 days, that gives a very accurate prediction of a due date, within 3 days.