The fetal abdominal circumference (AC) is an important measurement when dating a pregnancy. Measuring the AC too small (the more common error) can potentially have catastrophic consequences to the course of the pregnancy.
To accurately measure the abdominal circumference, certain criteria have to be satisfied:
the abdominal image should be a true axial (i.e. no foreshortening)
the full circumference of the abdominal wall should be visible (ideally)
the measurement should be around the skin surface (not deep to the skin)
three landmarks should be visible (depending on fetal age)