Introduction:  Chorioamnionitis is a common cause of second trimester pregnancy loss, usually due to ascending infection. This study investigates the prevalence and bacteriology of chorioamnionitis in cases of spontaneous pregnancy loss in previable gestations (16–22 weeks).
Methods:  Fetal losses between 16‐ and 22‐week gestation were identified from the institutional database over a three‐year period. Cases with an autopsy were identified, pathology reports reviewed, and maternal features noted (clinical symptoms, blood count and vaginal culture results). Second trimester medical termination for fetal abnormality during the same time period served as controls for the confounding influence of labour.
Results:  A total of 101 cases of spontaneous non‐anomalous non‐macerated fetal losses and 103 control cases of induced loss for fetal anomaly were identified. Median gestation of cases was 19 weeks (interquartile range (IQR) 17, 21) and of controls was 20 weeks (IQR 19, 21). Maternal white cell count was higher in cases (median 13.6 IQR 10.8, 16.6) than in controls (9.9 IQR 7.6, 11.5) (P < 0.01). Seventy‐eight (77.2%) of 101 cases and no controls had histological chorioamnionitis. A fetal reaction was identified in 48.7% of cases with chorioamnionitis, and the frequency of fetal reaction increased as gestation advanced (5.3% at 16‐week gestation vs 33.3% at 22‐week gestation). In cases with chorioamnionitis 36/76 (47.4%) were culture positive, whereas 4/25 (16%) without chorioamnionitis were culture positive. Conclusion:  In otherwise normal fetuses, chorioamnionitis is a common finding in mid‐trimester pregnancy loss. Routine culture methods have a low sensitivity for isolation of the causative micro‐organisms. This inflammatory process seems to predate the onset of labour and appears a primary mechanism in the aetiology of such losses.