How accurate are ultrasound scans in pregnancy?

Depends: The precision of an ultrasound depends somewhat on the equipment, the technician, the interpreter & the reason for study. If the scan is done to verify the placental location, fetal position & head size(to verify dates), simple equipment works ok. Specific things about baby like heart anatomy, facial features and the like, require a skilled technician using special equipment & interpretation.

Earlier the better: Ultrasound becomes less accurate as the pregnancy progresses.