The enlarged uterus may compress the abdominal and pelvic veins known as the inferior vena cava and the common iliac veins, respectively. Even before the uterus begins to enlarge significantly, hormonal factors cause the veins to become more compliant. Together with the increased venous pressure that occurs later in pregnancy, these factors cause significant venous distension and exaggerate the factors that predispose the patient to vein dysfunction.
Varicose veins appear in 30% of patients during a first pregnancy. The dysfunction becomes more severe with 55% of patients having problems after a subsequent pregnancy.