Background: Preterm prelabor rupture of membranes (PPROM) is responsible for approximately one third of preterm deliveries. Prenatal sonographic (US) assessment of the fetal thymus has become available. This research was done to investigate the role of ultrasonographic assessment of fetal thymus size in pregnancy associated with preterm premature rupture of membrane and its relationship with fetal outcome. Methods: This prospective controlled observational study was carried out on 60 cases with maternal age from 18 to 40 years old, gestational age between 24 and 36 weeks, singleton pregnancy and associated with PROM. Cases were subdivided into two groups Group A: 30 cases with PROM, Group B: 30 normal at the same gestational age. All cases were subjected to history taking, obstetric history, maternal medical history, investigation studies and ultrasound study. Results:The amniotic fluid index had a significant decrease in group A compared with B, and the gestational diabetes mellitus had a significant increase in group A compared with B. The small fetal thymus size had a significant increase in group A compared with B (p =0.013). The Puerperal endomyometritis had a significant increase in group A (30%) compared with B (0%). The Apgar score at 5 mins (p<0.001) and Apgar score (p= 0.042) at 10 mins were significantly decreased in group A compared with B. Conclusions: In pregnancy associated with preterm premature rupture of membranes, small fetal thymus might be associated with histologic chorioamnionitis, higher rate of perinatal mortality and short-and long-term neonatal morbidity however many cases remained clinically silent.