CONTRACTILE ACTIVITY OF THE UTERUS PRIOR TO LABOR ALTERS THE TEMPORALORGANIZATION OF SPONTANEOUS MOTOR-ACTIVITY IN THE FETAL SHEEP

Contractile activity of the uterus before the onset of labor (uterine contractures) has been described in a number of species and provides a powerful source of repeated stimulation for the fetus throughout much of gestation. To understand how fetal behavior responds to this dynam ic aspect of the intrauterine environment, we investigated the effects of uterine contractures on the temporal organization of spontaneous m otor activity in the fetal sheep during the last fifth of gestation. E leven fetuses were instrumented on 113-116 days of gestation (dGA). El ectromyogram (EMG) activity was recorded from flexor and extensor musc les in the fetal forelimbs and hindlimbs, and from the uterus. Pooled limb EMG activity from 2300 hr to 0700 hr on 118, 125, 132, and 139 dG A before, during, and after uterine contractures was spectral analyzed to detect and quantify the cyclic organization in fetal motor activit y. There was strong evidence of cyclic organization in fetal motor act ivity (CM) at each gestational age, similar to what has been described in the fetal rat and human. There was no evidence of developmental ch anges in the baseline spectral measures of CM. The most prominent feat ure of the response of CM to uterine contractures was a transient decr ease in irregularity at 118-132 dGA. The strength of CM increased duri ng contractures at 125 and 132 dGA, and a slight acceleration of CM du ring contractures was detected at 118 and 139 dGA. The results demonst rate that the stimulation associated with contractures influences an i mportant source of complexity in early behavioral organization. The re sults are consistent with speculation by others that uterine contractu res might induce transient cerebral hypoxemia in the fetus, and sugges t that conditions established in the first few minutes of sustained ut erine activity constitute the effective perturbation of CM. (C) 1996 J ohn Wiley & Sons, Inc.