"Although it is helpful in estimating gestational age, identifying twin pregnancies, and detecting genetic anomalies, the American College of Obstetrics and Gynecology (ACOG) position is thatroutine ultrasonographic screening during pregnancy is not mandatory. They deem routine use reasonable when requested by a patient.
...However, some expectant couples have followed the lead of actors Tom Cruise and Katie Holmes and purchased (for costs ranging between $15,000 and $200,000) their own ultrasound machines, which they use daily."
I know some natural practioners recommend limiting ultrasounds largely because we dont' know what they do. An OB friend also forwarded a study suggesting a correlation between lots of ultrasounds and autism. Who knows?