Control of fetal breathing in the human fetus between 24 and 34 weeks' gestation.

The effect of induced maternal hypocapnia and hypercapnia on fetal breathing movements was studied in 30 healthy pregnant women between 24 and 34 weeks' gestation to determine whether gestational age influences the fetal respiratory response to alterations in carbon dioxide levels. The percent time of fetal breathing movements correlated significantly with maternal end-tidal PCO2, increasing with maternal breathing of 2% and 4% carbon dioxide and decreasing with maternal hyperventilation for each of the three gestational age groups studied. However, the slope of this response for the 28- to 30-week group (3.29) and for the 32- to 34-week group (3.66), although similar to that of the term fetus, was significantly greater than that for the 24- to 26-week group (1.18, p less than 0.001). We conclude that the carbon dioxide level in the preterm fetus (as in the term fetus) is an important stimulus for the generation of respiratory movements. However, a developmental change is evident; the 24- to 26-week fetus demonstrates a decreased respiratory response to tonic carbon dioxide input. This may account for the decreased incidence of fetal breathing movements in the fetus of younger gestational age.