Biorhythms of the primate uterus (Macaca mulatta) during labor and delivery. 
The association between the documented spontaneous biorhythms of uterine activity, uterine artery blood flow, and aortaic blood pressure and the processes of labor and delivery was explored in 17 nonmedcated rhesus monkeys for 18 pregnancies.  The characteristic circadian variations of higher hourly average values of uterine activity during periods of light than during periods of darkness were incorporated in a statistically distinct, predictable, and repetitive manner into the events of parturition.  Variations in uterine artery blood flow reflected changes in intra-amniotic pressure.  In those animals in which delivery occurred at night, a shift in maximal values of uterine activity from periods of light to periods of darkness occurred 48 to 60 hours prior to delivery.  Progressive linear increases in hourly average values of frequency of contraction and aortic blood pressure consistently began 24 to 36 hours prior to delivery.  These changes suggest that specific, directly related prodromal events occur within the maternal organism hours before parturition in the nonhuman primate.