Small fetal thymus and adverse obstetrical outcome: a systematic review and a meta-analysis

Introduction: The aim of this study was to explore the association between small fetal thymus on ultrasound and adverse obstetrical outcome. Material and methods: Medline, Embase, Cochrane and Web of Science databases were searched. Primary outcome was the risk of preterm birth before 37 and 34 weeks of gestation in fetuses with, compared to those without, a small thymus on ultrasound. Secondary outcomes: occurrence of chorioamnionitis, intrauterine growth restriction, neonatal sepsis, gestational age at birth, birthweight, neonatal morbidity and preeclampsia. Results: Twelve studies including 1744 fetuses who had ultrasound assessment of thymus during pregnancy were included. Women with preterm premature rupture of the membranes or with preterm labor were at higher risk of preterm birth before 37 weeks (p = 0.01), or before 34 weeks (p < 0.001) for fetuses with a small fetal thymus compared to those without a small thymus, and the risk of chorioamnionitis was higher when the thymus was small (p < 0.001). Fetuses with small thymus were not at higher risk of intrauterine growth restriction (p = 0.3). A small thymus increased the risk of neonatal sepsis (p = 0.007) and morbidity (p = 0.003), but not the risk of preeclampsia (p = 0.9). Conclusions: A small fetal thymus is associated with a higher risk of preterm birth, chorioamnionitis, neonatal sepsis and morbidity, but not with intrauterine growth restriction and preeclampsia.