Circadian incidence of labor onset hour in preterm birth and chorioamnionitis.

Normal labor begins most frequently between midnight and 2 AM, presumably because of an unidentified circadian labor-activating mechanism. We used records of the Collaborative Perinatal Project to determine whether such a circadian mechanism is present with intrauterine growth retardation (IUGR) or preterm birth, and whether chorioamnionitis is associated with alternative patterns. In the absence of chorioamnionitis, both preterm births and term births with IUGR demonstrated nocturnal labor onset distributions like those of control term deliveries, which had a peak incidence at 1:45 AM and an amplitude of 35%. With chorioamnionitis, in contrast, there was a different circadian distribution of term labor onset hour, with a peak at 7:45 PM and a 32% amplitude. A similar trend was found among preterm births. These data suggest the presence of the normal nocturnal labor-activating mechanism in both growth-retarded term deliveries and a defined subset of preterm deliveries. Chorioamnionitis appears to be associated with a prominent, unidentified alternative labor-activating mechanism that is maximal in the evening.