OBJECTIVE The purpose of this study was to summarize the published data on the fetal thymus, the fetal thymic involution and the possible use of the measurement of the fetal thymus in the clinical practice. METHODS Review. METHODS Department of Obstetrics and Gynecology Medical Faculty Charles University Hradec Kralove. METHODS We summarized published data on the fetal thymic involution and relationship between the sonographic fetal thymus size and the presence of an intrauterinne infection in patiens with preterm labor or preterm prelabour rupture of membranes. CONCLUSIONS The present review summarizes the results of clinical and experimental research on the fetal thymic involution.