The fetal breath cycle.

Fetal breathing movements may be timed precisely from the umbilical vein flow velocity profile recorded using Doppler ultrasound. Serial studies were performed on eight normal fetuses from 28 weeks pregnancy until delivery. Measurements were made of the inspiratory time, total breath time and breath amplitude. There was a clear change in the pattern of fetal breathing with advancing gestation. The inspiratory time lengthened and the amplitude of each breath increased. The variability of these measures decreased. The timing equation ti/ttotal lengthened. The area of change in the umbilical vein sonogram was calculated as a measure of respiratory work and this increased with gestation. It is suggested that the changing pattern of fetal breathing movements reflects changes in central control of breathing in the fetus and parallels observations in the newborn.