Introduction Renal vein thrombosis (RVT) in neonates is a rare condition that carries low mortality but high morbidity. Aetiology isn’t fully understood; predisposing factors are dehydratation, sepsis, asphyxia, polycythemia, maternal diabetes, traumatic delivery, congenital renal vein defects, umblical catheterisation, prothrombotic conditions.

Case presentation 36 week baby was born by C/S to 32 year-old, gravida 2 mother. The only prenatal risk was gestational diabetes. She was born early because of fetal distress. Evaluation of the infant for jaundice revealed left flank mass and edematous left leg in the second day of life. No effusion was detected in the joints of hip and knee. Lower extremity doppler USG was normal. Abdominal USG showed enlarged left kidney. Doppler USG showed thrombus in the inferior venae cavae, extending to left renal vein, main iliac veins, right external iliac vein. Right renal vein drained to retroperitoneal collaterals. Abdominal tomography confirmed USG. Retrospectively antenatal history revealed enlarged left kidney determined by USG performed right before birth. Based on retroperitoneal collaterals and prenatal USG we think RVT probably began in utero. There was no evidence of sagittal sinus thrombosis. The neonate was treated with LMWH. The results of clotting studies of mother were normal; heterozygot mutation of factor V Leiden and MTHFR gene were found in the baby. Follow-up renal scan at 3 months documented a non-functioning left kidney.

Conclusion In neonatal period; when renal vein thrombosis and disseminated thrombosis is detected in the absence of other risk factors, prothrombotic conditions should be searched.