Fetal age and patterns of human fetal breathing movements.

Human fetal breathing movements have been studied serially from 20 weeks to the onset of labor in eight normal primigravid patients. Real-time ultrasound was used and the events of the study period were analyzed visually and by computer processing of a punched paper tape record. Studies were performed before and after a maternal glucose load. The pattern of fetal breathing movements changed with increasing fetal age. The breathing rate decreased and became more regular toward term. The form of each breath altered with gestational age. Prior to 32 weeks the breath time was short. Between 32 and 36 weeks a breath characterized by a long inspiratory phase with multiple augmenting movements was seen. After 36 weeks the breath was more uniform. A definite periodicity was seen in the last 2 weeks of pregnancy, with episodes of shallow, more regular breathing and a more variable pattern. Glucose intake did not alter these patterns.