This is a typical grey-scale (typically in 256 shades of grey levels) B-scan sonogram image of the fetal abdomen at the level where the circumference will be measured. Notice that in an ultrasound image solid areas are depicted in 'white' and the fluid areas in 'black'. In this typical cross-sectional slice of the abdomen the stomach and umbilical vein (left side of the image) are seen. An ultrasound image is always a slice of the part of the fetus that one tries to see. When similar images are seen on the screen during a scanning session, mothers often thought this is the face of the fetus (with the two eyes, nose and mouth) as they would tend to wrongly perceive the image as a picture taken with a camera in front of the fetus.

A single beam in an ultrasound scan can be used to produce an M-mode picture where movement of a structure such as a heart valve can be depicted in a wave-like manner. This is useful in assessing rates and motion.

An A-mode picture is now obsolete in obstetric scans. Wave spikes are represented when a single beam passes through objects of different consistency and hardness. The distance between these spikes ( for example A and B ) can be measured accurately when on-screen calipers are not available in the old days. At that time when ultrasound scanning was first invented the scans were single-pass scans and were known as 'static scans' as compared to the multi-pass realtime scans we have now.