Q & A: Ultrasound Accuracy

Q: Is it possible that my scan at 5 weeks & 5 days showing nothing but a sac could really be a 8-9 week gestation? Making my 11 week scan really a
13-14 week fetus? -Rachel

A: In theory, no. In reality, I have had personal experience with the following:

- an ultrasound at 6 weeks saying that a women had a miscarriage...this woman is still pregnant today
- a first trimester ultrasound ruling out pregnancy and ectopic pregnancy....that woman later had an ectopic pregnancy that burst and she had emergency surgery
- several women who have had ultrasounds show significant anomolies or conditions in their babies...that went on to give birth to babies who did not have these conditions

Although I have not personally seen it, it is even possible to have a twin that is hiding behind it's sibling on the ultrasound be a surprise at the time of birth!

Ultrasound is a tool. It isn't perfect, but it is frequently useful, so we use it. (Did I mention it isn't perfect?)

In theory, first trimester ultrasound dates a pregnancy to an accuracy of +/- 3 days. In reality, it can be wrong.

If you know when you conceived, that is the best way to date a pregnancy. Otherwise, using a combination of LMP (the first day of your last period) and ultrasound is a good second best.