Ultrasonographical evaluation of the incidence of simultaneous and independent movements in twin fetuses.

Fetal movements (FM) may be spontaneous, originating in the fetus itself, or evoked by external stimuli. The relative rate of spontaneous and evoked FM has not yet been documented. As it is impossible to differentiate in a singleton pregnancy between these types of movements - the natural model of twin pregnancy was used. The FM of 10 normal twin pregnancies between 33 and 39 weeks of gestation were studied by two realtime ultrasounds, and were operated by two ultrasonographers over a period of 10 min. The mean number of independent FM of each fetus which occurred while the other fetus was resting was 7.9 FM and 7.7 FM, respectively (average 7.8 FM). A mean of 2.5 FM occurred simultaneously. The rate of independent FM was 75.7% and that of simultaneous FM was 24.3%. Vibroacoustic stimulation applied to the maternal abdomen evoked simultaneous movements of both fetuses in all cases. It is suggested that the independent FM may represent spontaneous FM and that the simultaneous FM may indicate the presence of an external stimuli and are therefore evoked responses.