Women who use the standard form of birth control pill may have a much higher than average risk of dangerous blood clots if they also have a vein malformation seen in a quarter of the population, California researchers say.

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.

Some 12 million women in the U.S. use the "combined" form of birth control pill, which is already known to increase a woman's risk of blood clots.

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.

In the study, Hofmann's group looked at 35 women with deep vein thrombosis and 35 women without the condition for comparison. They found that left-side common iliac vein stenosis raised a woman's clot risk 3.5 times. Combined oral contraceptives raised clot risk nearly 5 times. And the combination of the two raised the risk 17.7 times.

For the study, the Stanford group used CT or MRI scans to evaluate women with deep vein thrombosis. They compared them to an equal number of women of the same age who came to the emergency room with abdominal pain.

In total, 37 percent of the women with deep vein thrombosis and 11 percent of the control subjects used the combined type of birth control pills. Among thrombosis cases, 63 percent also had other clot risk factors, such as pregnancy or a clot-prone condition called thrombophilia, compared to 23 percent of the controls.

Among women with deep vein thrombosis and left common iliac vein stenosis, the degree of narrowing of the vein was, on average, 70 percent, compared to 56 percent blockage in controls.
For the women in general, every one percent increase in iliac vein stenosis above 70 percent boosted the risk of thrombosis by five percent. But for women using the Pill, every one percent increase in stenosis raised the risk of thrombosis about 500 percent.

Given the risks, the authors caution doctors that, "If a young woman using combined oral contraceptives develops left-sided deep vein thrombosis in the absence of other risk factors, the possibility of common iliac vein stenosis should be considered."