We’ve all seen our blood clot as a healthy response to a superficial scratch on our arm or leg, but what happens when a clot occurs within a vein? When this unusual event happens in the brain, it may lead to a specific kind of stroke known as cerebral venous thrombosis (CVT). And now, a new study from the Academic Medical Centre in Amsterdam has linked CVT risk to oral contraceptive use among obese women.

When a clot forms inside the brain, blood can begin to leak into cerebral tissues, forming a hemorrhage. According to Johns Hopkins Medicine, this chain of events may ultimately lead to CVT, which damages not only the brain but other parts of the central nervous system. Though CVT is rare — it accounts for 0.5 to 1 percent of all strokes — one well known risk factor is oral contraceptives.

However, no one has assessed the potential for an increased risk among women who are both obese and taking birth control pills. This is not an idle question, the authors suggested, since past studies have shown obesity will amplify people’s risk of deep vein thrombosis, a type of blood clot that occurs in the leg, and pulmonary embolism, a dangerous blood clot that forms in the lungs.

For the current study, Dr. Jonathan M. Coutinho and his coauthors studied the medical records of 186 patients with CVT from two hospitals. Comparing these case histories to those of 6,134 healthy “controls,” the Dutch team drew some surprising conclusions. They found the patients with CVT were more likely to be younger, female, using oral contraceptives, or having a history of cancer when compared to the other study participants.

Further analyzing the numbers, the authors suggested it is obesity (a body mass index of 30 or higher) that confers this increased risk of CVT among women taking oral contraceptives.

Specifically, the researchers found a nearly 30-fold increased risk of CVT among obese women on oral contraceptives compared to normal weight (BMI of less than 25) women who weren’t on the pill. Overweight (BMI of 25 to 30) women on the pill also showed an increased risk for the rare stroke. Most importantly, the researchers found no relationship between CVT and obesity in men or women who didn’t use hormonal contraceptives, indicating that the pills could be at fault.

In an accompanying editorial, Dr. Chirantan Banerjee of Medical University of South Carolina noted that initially scientists believed only estrogens in oral contraceptives increased clot risk. Yet over time, researchers also linked progestins, such as drospirenone, sold under the brand name Yasmin in the United States, and desogestrel, third-generation birth control pills, including the brand Cyclessa, to venous thromboembolism, which is similar in the way it progresses to CVT.

Banerjee concluded, “Better counseling and education of obese women, informing them of the increased risk would be prudent, as would be consideration of alternate non-hormonal oral contraceptive options.”

Source: Zuurbier SM, Arnold M, Middeldorp S, et al. Risk of Cerebral Venous Thrombosis in Obese Women. JAMA Neurol. 2016.