A study of African American women shows that use of birth control pills and other forms of hormonal contraception, and possibly the presence of the sickle cell trait, increase the risk of venous thromboembolism (VTE) - blood clots that form in the veins.

An inherited condition, sickle cell trait occurs in people with one copy of the sickle cell gene, meaning some oxygen-carrying protein of the blood is abnormal, but individuals do not develop full-blown sickle cell disease. Sickle cell trait and sickle cell disease occur most often in African Americans.

“There is a suggestion” that sickle trait may increase African American women’s risk of VTE “beyond that which is due to the contraception itself,” Dr. Nigel S. Key from University of North Carolina School of Medicine, Chapel Hill, told Reuters Health.

“However, these data need confirmation, and if true, the absolute risk of thrombosis is very small.”

Among a group of 196 African American women, Key’s team found that hormonal contraception was associated with a 2.8-fold increased risk of VTE, which increased to 3.8-fold after adjustment for household income.

Among current users of oral contraceptives, the age- and income-adjusted risk of VTE was increased 3.4-fold compared with women who were not on hormone contraceptives, the researchers note, and the increased risk was higher among women with sickle cell trait (6.7-fold increase) than among women without sickle cell trait (2.6-fold increase).

“The present study provides very persuasive evidence that hormonal contraceptive use increases the risk of VTE among African American women, as is the case among white women,” the investigators conclude in the American Journal of Obstetrics & Gynecology.

“We believe this is the only study that evaluates the hormonal contraception/VTE association among African Americans,” they add.

“Additional research,” Key told Reuters Health, “is clearly needed to understand not only the epidemiology and magnitude of risk, but also the mechanisms that may explain this finding.”

SOURCE: American Journal of Obstetrics and Gynecology, June 2009.