Background: Changes in thymus size and histopathology have been observed both in animal models of intrauterine growth restriction (IUGR). The aim of the present study was to evaluate the size of the fetal thymus by sonography in pregnancies with IUGR and to search for a possible relationship between a fetal thymus size and adverse perinatal outcomes. Methods: This prospective observational study was carried out on 100 participants who were divided into two groups. Group A: Study group which include 50 patients with IUGR. Group B: Control group which Include 50 normal patients with appropriate gestational age. All patients were subjected to: History taking: (Personal, Obstetric History, Maternal Medical History) and trans-abdominal ultrasound. Results: IUGR group show statistically significant decrease in the estimate of fetal weight (EFW) compared to the control group (P<0.05). Doppler study of umbilical artery shows significant increase of (pulsatility index (PI), resistance index (RI) and systolic/diastolic (S/D) in IUGR group when compared to control group. Doppler study of middle cerebral artery (MCA) shows significant increase in (RI, SD) in IUGR group when compared to control group while PI doesn’t show significant difference between two groups. Thymus size decrease in IUGR group when compared to the control group. IUGR group had low survival and lower APGR Score when compared to the control group. Correlation between thymus size with the studied doppler parameters and pregnancy outcome in the current study. Umbilical Doppler RI, PI and SD showed statistically significance in this study (P<0.05) and this means that the blood flow in the umbilical arterial (UA) is important for the fetus. As regard the MCA RI and SD Doppler, they show statistically significance in this study (P0.05). Conclusions: IUGR is associated with small thymus and small fetal thymus may be an early indicator of adverse perinatal outcomes in pregnancies complicated by IUGR.