What’s the accuracy of an ultrasound for a baby?

For what purpose. It depends on the question it is being asked to answer.
Varies widely... Published experience proves that the detection rates for various fetal anomalies vary widely by organ system (brain spine urinary tact more readily detectable than heart skeleton or intestinal tract) and size/location of screening center (larger urban university-affiliated units much more accurate than smaller suburban or rural private units). Other factors: maternal obesity fibroids fluid disorde.