Up to 25 percent of the healthy population has a narrowing, known as stenosis, in the left common iliac vein (one of two major veins deep in the pelvis that return blood from the lower body to the heart), according to Dr. Lawrence Hofmann and his colleagues at Stanford University School of Medicine.

In their study, published in the American Journal of Obstetrics and Gynecology, having a "left common iliac vein stenosis" and taking so-called combined oral contraceptives - the kind containing both the hormones estrogen and progestin - multiplied a woman's risk of deep vein thrombosis nearly 18 times compared to women with neither risk factor.

Sometimes known as economy class syndrome," a deep vein thrombosis is a blood clot - often formed in veins of the lower legs -- which is potentially dangerous because it can travel to the heart and lungs and cause serious harm.

In general, the researchers note, among every 10,000 young women who are not taking oral contraceptives, about 1 to 3 will have a deep vein thrombosis every year. For young women who've been taking the Pill for a year, that risk goes up six-fold.