The prevalence of chronic deciduitis in cases of preterm labor without clinical chorioamnionitis.

Preterm labor is a major cause of perinatal mortality and morbidity, and in approximately 30% of cases a clinical cause is not identified. Acute chorioamnionitis is found histologically in a significant percentage of placentas from preterm deliveries, and the mother is often asymptomatic. Although such subclinical acute chorioamnionitis is known to play a role in preterm labor, this study explores the hypothesis that chronic deciduitis with plasma cells is seen more frequently in cases of preterm labor than in control placentas. Thirty-nine singleton placentas from patients with idiopathic preterm labor were examined microscopically and compared in a blinded fashion with 39 gestational age-matched control placentas. Cases of clinical acute chorioamnionitis and known chronic maternal diseases were excluded. Thirty-nine control singleton placentas were obtained from patients undergoing induction of labor for fetal structural abnormalities, excluding aneuploidy. The presence or absence of acute chorioamnionitis, acute fetal inflammatory response, chronic deciduitis, chronic villitis, infarction, and decidual vasculopathy was noted. Immunohistochemical staining was undertaken to further define leukocyte subtypes. Forty-one percent of cases and 15% of controls showed chronic deciduitis (P = 0.022). Forty-six percent of cases and 18% of controls showed histologic acute chorioamnionitis (P = 0.015). There were 8 cases demonstrating acute fetal inflammatory response but only 1 control (P = 0.029). Little difference was seen in the distribution of lymphocyte subsets between cases and control placentas. Our findings suggest that chronic deciduitis plays a role in the etiology of some cases of preterm labor.