Can you tell by ultrasound if a baby has congential heart defects?

Yes: The ultrasound technology has advanced significantly to be able to detect heard defects pretty reliably now a days. Occassionally small defects are not seen. There is also some variation based on how good the ultrasound technician is.

Depends: The results of a prenatal ultrasound depend on the sensitivity of the equipment used, the technician and the person interpreting the study. Some studies are fine for locating the fetal position, placental location and estimating fetal size, but little more. Most newer machines are capable of picking up most defects but a few will not be evident. Newborn exam and postnatal study may be needed.