Incidence of spontaneous and evoked fetal movements.

Fetal activity is composed of fetal movements (FM), which are either nonreflexive, originating in the fetus itself, or evoked reflexive FM which are stimulated by the fetal environment. The relative ratio of the spontaneous to evoked FM has not yet been documented. Since it is impossible to differentiate in a singleton pregnancy between these two types of FM on the basis of maternal perception alone, the natural model of twin pregnancy was used. It has previously been shown that the number of FM in a singleton pregnancy is similar to that of each individual fetus in a twin pregnancy of the same gestational age. It was assumed that the factors which cause spontaneous and evoked FM are similar in singleton and twin pregnancies. The fetal activity in 44 twin and 76 singleton pregnancies between 28 and 39 weeks of gestation was assessed during 20 min of nonstress test recording. The means of FM in twins were significantly higher than those in singleton pregnancies. The rate of the spontaneous FM was calculated to be 50-59% of all FM in singleton pregnancies and that of the evoked FM 41-50%.