Ever feel like having your period messes with your mind? Well, a new study finds that it does.

Fluctuations in hormone levels can reshape the brain, according to a small study on 30 women. The researchers captured data on the participants’ brain changes throughout the menstrual cycle. The results suggest that brain changes don’t only occur in parts of the brain that are linked to menstruation—but are more widespread. 

“These results are the first to report simultaneous brain-wide changes in human white matter microstructure and cortical thickness coinciding with menstrual cycle-driven hormone rhythms,” the researchers wrote in the preprint report, which has yet to be peer-reviewed.

The findings are fascinating. The team conducted MRIs on each woman during three menstrual phases. At the same time, the team measured the participants’ hormone levels.  As hormones shifted, so did gray and white matter volume (and cerebrospinal fluid, too). White matter, which is a network of fibers that transfer information between areas with gray matter, is known to shift as hormones do. Gray matter is tissue in the brain and spinal cord that involves mental functions, memory, movement, and emotions.

If you’re regularly experiencing brain fog, mental fatigue or mood swings during your period (most notably during your luteal phase, which is associated with PMS), this could be a possible physiological link as to why. Taking it one step further, it’s also possible that these brain changes could be one reason why some types of mental health conditions, like anxiety and depression, seem to worsen around your period.

“Cyclic fluctuations in hypothalamic-pituitary-gonadal-axis hormones exert powerful behavioral, structural, and functional effects through actions on the mammalian central nervous system,” the authors wrote. “Yet, very little is known about how these fluctuations alter the structural nodes and information highways of the human brain.”

Just before the women ovulated, when 17β-estradiol and luteinizing hormone went up, the participants showed changes in white matter, meaning that their brains were transferring information more quickly than usual. Follicle-stimulating hormone (FSH), which rises before ovulation to stimulate ovary follicles, was linked with thicker gray matter. Progesterone, which rises after ovulation, was associated with increased tissue and lower amounts of cerebrospinal fluid.

The researchers didn’t report on functional consequences of these brain changes—instead they only documented them. But the changes they found could have “implications for hormone-driven alterations in behavior and cognition,” they wrote. It’s clear that more research is needed, but the results are an early step in paving the way for more understanding about how the menstrual cycle impacts women’s mental health.  

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