Cyclic motor activity in the human fetus after midgestation.

The spontaneous movements of human fetuses were studied longitudinally between 21 and 41 weeks of gestation. At each age, spectral analysis of the fluctuations in fetal movement revealed strong cyclic patterns. The frequency of the dominant motor rhythms ranged from .13 to 1.91 cycles per minute. The strength of oscillations between .18 and 1.02 cycles per minute increased slightly with gestation, but the frequency of the dominant motility cycle, the width of the corresponding spectral peak, and the incidence of weaker (usually faster) rhythms did not change. Neuromuscular activity is known to serve important developmental functions in the embryo and fetus. The ubiquity and relative consistency of cyclic motility in the last half of human gestation, and the existence of similar periodicity in other species, suggest that the cyclic activation of developing motor circuits may possess particular adaptive value.