Fetal thymus size as a predictor of chorioamnionitis in women with preterm premature rupture of membranes.

OBJECTIVE: Emerging evidence indicates that chorioamnionitis is associated with a significant decrease in thymic size at birth in very low birth weight (VLBW) preterm infants. The aim of this study was to determine whether decreased fetal thymus size is associated with histological or clinical chorioamnionitis in patients with preterm premature rupture of membranes (PROM).
METHODS: Twenty-one patients between 24 and 35 weeks of gestation with preterm PROM were included. Serial ultrasound examinations were performed during the latency period, and measurements of the fetal thymus size were obtained. Small thymus was defined as a thymus perimeter < or = 5th percentile according to a fetal thymus nomogram, which was based on measurements of 403 fetuses. Diagnosis of chorioamnionitis was made using neonatal clinical parameters and histological examinations of the placentas.
RESULTS: In our study 13 patients presented with thymus size below the 5th percentile. Among the 13 patients with small thymus, nine (69%) had clinical or histological findings consistent with the diagnosis of chorioamnionitis. All eight women with a normal-sized thymus had no evidence of clinical or histological chorioamnionitis. Fetal thymus perimeter < or = 5th percentile yielded a sensitivity of 100%, specificity of 66.7%, a positive predictive value of 69% and a negative predictive value of 100% for identifying chorioamnionitis in patients with preterm PROM.
CONCLUSIONS: Fetal thymus size is decreased in women with preterm PROM and chorioamnionitis. Measurement of the fetal thymus might allow an early diagnosis of chorioamnionitis in cases of preterm PROM. Normal thymus size might be used to rule out latent intrauterine infection.