Background: Preterm premature rupture of membranes (PPROM) is an important risk factor for early neonatal sepsis which remains a feared cause of morbidity and mortality in the neonatal period. This study aimed to assess the role of ultrasound measurement of fetal thymus measures as a marker of subclinical chorioamnionitis in preterm premature rupture of membranes. Methods: A prospective analytical study which included 206 pregnant women who fulfilled the inclusion criteria in department of obstetrics emergency hospital and outpatient antenatal care unit faculty of medicine Zagazig University in the period from October 2017 to February 2020. Patients were divided into 2 groups; Control group: 103 cases of uncomplicated pregnancy between 24-36 weeks gestational age undergoing ultrasonographic fetal anomaly scan and the fetal thymus measures was evaluated. Study group: 103 cases presented to our hospital by preterm premature rupture of membrane between 24-36 weeks gestational age. All patients underwent a detailed sonographic evaluation to assess fetal biometry, amniotic fluid volume and to exclude major structural fetal anomalies. Results: 68.9% of studied cases had positive histological examination for chorioamnionitis versus 0% of their controls. The ability of small thymus transverse diameter to detect subclinical chorioamnionitis occurrence among PROM females was 100%, while it could exclude 91.5% negative cases among truly negatives examined. With accuracy as a predictor tool of 94.2%. Conclusions: Assessment of the decrease in fetal thymus diameter by serial ultrasonographic examinations is a promising prenatal method in the prediction of intra-amniotic infection and subclinical chorioamnionitis in cases of PPROM.