The rate and regularity of breathing movements in the normal and growth-retarded fetus.

The rate and regularity of fetal breathing movements (FBM) were determined in 14 women with uncomplicated singleton pregnancies, eight of whom were between 30 and 33 weeks gestation and six between 37 and 40 weeks gestation. Similar observations were made in 19 women with pregnancies complicated by severe intrauterine growth retardation, 11 of whom were between 30 and 33 weeks and eight between 37 and 40 weeks. In normal pregnancy recordings of breath-to-breath intervals showed that FBM became more regular with advancing gestational age, and the rate [breaths/min, mean (SEM)] slowed from 57.2 (1.3) at 30-33 weeks to 47.9 (0.8) at 37-40 weeks. FBM in the growth-retarded group were regular at each gestation studied and the rate was even slower than in the normal group at term, being 41.9 (1.2) at 30-33 weeks and 41.1 (1.0) at 37-40 weeks. Hyperoxia and hypercapnia appeared to have no consistent effect on fetal breathing rate. Fasting for greater than 12 h considerably reduced the rate of FBM in the normal fetus but only marginally so in those with growth retardation. It is concluded that the pattern of FBM provides more information about the fetus than the amount of time spent breathing, particularly when growth is retarded.