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Simultaneous observations of the movements of opposite sides of the human fetal trunk were made by an ultrasonic device. Rhythmical excursions that were in time but out of phase were recorded from the lateral chest walls. These movements are considered to be the result of changes in the shape of the chest and are consistent with breathing movements. Large excursions of the antior abdominal wall compared to the chest movements suggest that the diaphragm shares in the breathing movement.

A new low-frequency Doppler shift signal associated with fetal breathing has been observed. This signal is out of phase with the previously described signal from the inferior vena cava and originates from the umbilical vein.