a superficial and a deep venous system. The superficial system lies above the muscle fascia in the subcutaneous tissues. The principal veins in the superficial system are the great saphenous vein ...

the communicating venous system, and the deep venous system. [38] The greater and lesser saphenous veins are the two major vessels that drain the subcutaneous tissues of the lower extremity.

While still rare, your risk rises when you're expecting — which is why it's so important to recognize the symptoms.