Are you asking for coding rules or coverage rules? If coverage, that's up to the individual insurance plan. If coding, the Introduction to that section states the code includes determination of number of fetuses and amniotic/chorionic sacs, measurements appropriate for gestational age, survey of intracranial/spinal/abdominal anatomy, 4 chambered heart, umbilical cord insertion site, placenta location and amniotic fluid assessment, and, when visible, examination of maternal adnexa.
If that is not the purpose of the ultrasound, or you need to check on something that was abnormal on the first one, there are other codes.
