A small, preliminary study suggests that a brain area called the hypothalamus appears to be about 6% smaller in women who use birth control pills.
But exactly what that means isn't yet clear. In this study, women on the pill had statistically significant increases in anger. Researchers also found a possible link with depression symptoms.
The good news: They didn't see any difference in women's mental performance.
And women who use the Pill shouldn't search for other forms of birth control based on these findings, researchers said.
"There isn't enough data here for anyone to worry," said study author Dr. Michael Lipton. "There's more than a 50-year history of birth control pills. We're not advising any changes (in your contraception) based on this preliminary finding."
Still, he added, there may be clinical consequences that this study was too small to find.
Lipton is associate director of the Gruss Magnetic Resonance Research Center at Albert Einstein College of Medicine, in New York City.
Located at the base of the brain, the hypothalamus produces hormones. It also helps regulate essential body functions like temperature, sleep and heart rate, the researchers said.
"There's a lot of stuff packed in there," Lipton said. "The hypothalamus covers a lot of basic body functions, like sleep regulation, reproductive regulation, ovulation, the menstrual cycle, sex drive, appetite, mood, reward-related behavior and water [balance]."
Lipton and his team were initially studying differences in concussion recovery in female and male athletes. During that research, they realized there was a difference in hypothalamus volume in the women taking birth control pills.
Fifty women were part of the study. Twenty-one used birth control pills.
The U.S. Centers for Disease Control and Prevention estimates that about 16% of women between 15 and 44 years of age take birth control pills. They are also used to treat other conditions, including irregular periods, cramps, acne, endometriosis and polycystic ovary syndrome.
The researchers only looked at birth control pills and not at other hormonal forms of birth control, such as under-the-skin implants or intrauterine devices. Because these forms of birth control release a steady, rather than a cyclical dose of hormones, their effects on the body or brain may be different, Lipton said.
Dr. Minisha Sood, an endocrinologist at Lenox Hill Hospital in New York City, said it's important to note the study's small size, and that participants may have already had mood disorders.
"This study is not likely to change my clinical practice in the immediate sense, though it may serve as a stepping stone to increase our understanding of the complex relationship between oral contraceptives and the brain," Sood said.
Christine Metz, a professor at the Feinstein Institutes for Medical Research/Northwell Health in Manhasset, N.Y., also pointed out that the research does not prove cause and effect. "That means it's too soon to know why these changes might have occurred," she said.
Metz added that it's also "important to note that changes in volume do not necessarily mean that there will be changes in brain function. With a smaller nose, do you smell less?"
She said she doesn't think this is something women need to be worried about. The study was small, and it's hard to distinguish the hypothalamus from surrounding brain tissue in imaging, Metz explained. Plus, the brain can change over time, so it's possible changes in hypothalamus size might be reversed or improved over time.