In pregnant patients, venous thrombosis usually affects the left-side iliofemoral deep vein. Vascular ultrasound (US) offers distinct advantages in confirming the clinical suspicion of disease or suggesting the need for further tests. The authors encountered three consecutive cases of antepartum proximal vein thrombosis during a 6-month period (0.6 case per 1,000 deliveries). Lack of venous flow accentuation and lack of diameter change at the common femoral vein during the Valsalva maneuver are indications for iliac venography and suggest the diagnosis of iliac vein thrombosis. In distal iliac and proximal femoral vein thrombosis, noncompressibility of the vein and the presence of echogenic material within are indicative of acute thrombus.