Eye movements in the human fetus and newborn.

The human oculomotor system becomes active long before birth. At about 16 weeks (post-menstrual age) the first slow changes of eye position can be observed with the aid of real time ultrasonography. At 18-20 weeks more rapid eye movements are detectable. There is a gradual decline in the percentage of time spent in rapid and slow eye movements from 32-40 weeks. These eye movements are organized in episodes of activity, divided by episodes of quiescence which are at first unrelated to cyclical changes in other variables (such as breathing movements, body movements and fetal heart rate patterns). At about 36 weeks gestation, REM and non-REM episodes become linked with other changes and then represent behavioural states identical to those seen in the newborn infant. This organization of behavioral states in the fetus is identical with that found in preterm infants of comparable age. Opening of eyelids is difficult to see before birth but the existence of episodes of wakefulness seems most probable. Hence fetal eye movements will be in part related to wakefulness. In the newborn at term, the interval distributions of REMs during sleep and of scanning eye movements during wakefulness are statistically different.