Cyclic motor activity in the fetal rat (Rattus norvegicus).

Rat fetuses were observed on Day 20 of gestation using a technique that permits direct observation of fetal behavior. The resulting time series was analyzed to assess cyclic organization in fetal movement. Fetal activity did not occur randomly but showed significant cyclic variation with a mean frequency of 0.61 cycle/min. This finding agrees with studies of human fetuses, which also exhibit short-period cyclicity in motor activity.