High- but not low-grade histologic chorioamnionitis is associated with spontaneous preterm birth in a Swedish cohort.

Objective: To examine whether different grades of placenta inflammation are associated with risk for spontaneous preterm birth, taking into consideration maternal and delivery factors. Placentas from spontaneous preterm births were compared with a control group from full-term deliveries.

Methods: Placentas from 98 full-term, 71 late preterm (gestational week 34-37), and 65 early preterm (gestational week 22-33) singleton deliveries were analysed from the Karolinska University Hospital in Stockholm. The placentas were examined for histologic chorioamnionitis (HCA) grade 1 (low) and 2 (high). Mother, child, and delivery parameters were collected from maternity centre and delivery forms.

Results: There was a relatively low incidence of HCA in the preterm groups (26.7% and 38.5% in the late and early preterm groups, respectively). HCA 2 was most common in the early preterm group and HCA 1 was most common in the full-term group. The odds of early preterm birth was lower for placentas with HCA 1 compared to HCA 2 (OR = 0.17) and higher for placentas with HCA 2 compared to no HCA (OR = 2.17).

Conclusions: Despite a relatively low incidence of HCA in the preterm groups, HCA 2 seems to be associated with early preterm birth whereas HCA 1 seems to be part of the full-term delivery.