Maturation of fetal breathing activity.

Spontaneous breathing movements are present during fetal life and they are important for normal development of the lung. Early in gestation they seem to represent free-running activity of the respiratory centers, not controlled by peripheral mechanisms but probably dependent on a tonic CO2 drive. Maturation of sleep states brings into play powerful brainstem inhibitory mechanisms which control FBM. At birth these mechanisms must be overridden to allow the establishment of continuous breathing.