Few things. Recent surgery, prolonged immobilization, long distance traveling, cancer, and smoking cause traditional DVTs. If bilateral veins are compressed, an organ like the uterus can compress the veins but you will probably suffer from large fibroids . Blood disorders or other abnormal masses can also cause a mechanical compression at a higher location than both those veins. Emboli can reach both lungs. .
Many things. These are two separate - but often related things. Thrombosis of the pelvic veins is a clot in the iliac veins. These are some of the largest veins and present the most serious short term and long term problems. A pulmonary embolus is when that blood break breaks free and goes to the lungs. Risk factors for DVT include immobilty, underlying malignancy, hypercoag state, and having surgery.
Embolism. Pelvic vein thrombosis is seen after childbirth, infection (pelvic inflammatory disease), malignancy, pelvic surgery, and extension of leg vein thrombosis into the pelvic veins. When the clots in the pelvic veins migrate into the lungs they are called pulmonary emboli, causing shortness of breath, chest pain, and when large, a drop in blood pressure and even death.