Many heterosexual couples that use oral contraceptives report having less sex than those who use condoms or other forms of contraceptives. Doctors have long believed that this is due to the hormonal imbalance caused by the pill, but a new study challenges this idea, potentially identifying the root cause. According to the results of the new study, oral contraceptives may shrink an area of the brain that controls sex drive.

Dr. Michael Lipton, professor of radiology at the Albert Einstein College of Medicine in New York, along with colleagues, performed brain scans on 50 women, 21 of whom were on the pill.

They noticed that the women who used oral contraceptives had a smaller hypothalamus — a small but crucial part of the brain responsible for regulating hormones. Damage to the hypothalamus is known to affect sex drive, mood, heart rate, and sleep cycles. A smaller hypothalamus doesn’t necessarily mean that it is damaged in any way, though.

“We found a dramatic difference in the size of the brain structures between women who were taking oral contraceptives and those who were not,” Lipton said.

Doctors have known for years that oral contraceptives can drastically alter a woman’s behavioral patterns, including the way they manage emotions, mood, learning, sex, attraction, and stress, among other things.

The reason why women on the pill have a lower sex drive is due to the hormone progesterone, which is dominant throughout the menstrual cycle, sending a message to the body that ovulation is not required.

Researchers also found a correlation between a smaller hypothalamic volume and more frequent bouts of anger and depressive symptoms.

Previous studies have found other links between using contraceptives and changes in various brain structures, such as the hippocampus and basal ganglia, as well as cortical thickness.

That being said, if you’re using oral contraceptives, now is not the time to panic. The authors of the new study note that their findings are preliminary and that they merely found an association between the pill and a smaller hypothalamus.

The sample size is also rather small, so the association might prove to be not as strong if more participants are included.

Even so, the findings are rather concerning and warrant more investigation. Among the 47 million U.S. women who currently use contraceptives, about 13% use birth control pills, according to a report by the U.S. Centers for Disease Control and Prevention. 

In other news this week, scientists at MIT have developed an oral contraceptive that only has to be taken once a month, which could reduce unintended pregnancies that result from forgetting to take a daily dose.

The preliminary study that found a link between oral contraceptives and a shrinking hypothalamus was presented this week at the annual meeting of the Radiological Society of North America.