The relationship between sonographic fetal thymus size and the components of the systemic fetal inflammatory response syndrome in women with preterm prelabour rupture of membranes

Objective To assess the relation between sonographic fetal thymus size and the components of fetal inflammatory response syndrome (FIRS) in women with preterm prelabour rupture of membranes (PPROM). Design Prospective cohort study. Setting University hospital from January through October 2006. Population Fifty-six women with PPROM. Methods In these women, fetal thymus perimeter was measured sonographically. At birth, cord venous plasma interleukin-6 (IL-6) level estimation and histopathological examination of the placentas and umbilical cords were performed. Main outcome measures Small thymus size (< 5th percentile for gestational age) and its association with FIRS. Results From the 56 women with PPROM, 54\% had chorioamnionitis (CA), 23\% had funisitis. IL-6 level was > 11 pg/ml in 52\% of women and > 18 pg/ml in 41\%. A small thymus was more associated with male fetuses, shorter preterm prelabour rupture of membranes delivery interval, higher IL-6 level, higher frequency of funisitis and CA. When data were regressed for confounding, only IL-6 level and funisitis remained significant independent factors that influence the thymus size. In the subset of women (n = 19) who delivered within 1 week of first measurements, a small thymus had sensitivity and positive predictive value of 93\%, specificity and negative predictive value of 75\% and accuracy of 89\% in the identification of FIRS (IL-6 > 18 pg/ml and/or funisitis). Conclusions An association exists between fetal thymic involution and components of FIRS in women with PPROM. Small fetal thymus size may be considered a reliable sonographic marker of fetal involvement in the inflammatory response.