The findings suggest that fluctuating hormones during the menstrual cycle alter brain structure in ways that impact brain function and lead to cognitive changes.
"We have known that levels of hormones, such as oestrogen and progesterone, change over the course of the menstrual cycle," said Dr Lisa Mosconi of New York University.
"And studies have looked at how these hormone levels can impact cognition, mood and behaviour. But until now, we did not have any insight into what might be the neurobiological foundation for these changes."
The researchers took monthly magnetic resonance imaging (MRI) scans of the brains of 10 women aged between 22 and 36, who had regular menstrual cycles.
MRI scans can give a detailed picture of brain structure. In this study, the scientists used a method known as voxel-based morphometry, which measures differences in the volume of different parts of the brain.
The results revealed that three areas of the brain changed in size over the course of the menstrual cycle:
* The left hippocampus: This area is important for memory. It was at its smallest during the luteal phase, which is the second half of the menstrual cycle, and at its largest during the follicular phase, the first half of the cycle.
* The right amygdala: This area is involved in emotion and mood regulation. It was also at its smallest during the luteal phase and at its largest during the follicular phase.
* The anterior cingulate cortex: This area is involved in a range of functions, including attention, planning and decision-making. It was also at its smallest during the luteal phase and at its largest during the follicular phase.
These findings suggest that fluctuating hormone levels during the menstrual cycle alter brain structure in ways that could lead to cognitive changes, including memory impairment and emotional reactivity.
"Our study provides the first evidence of structural changes in the brain over the menstrual cycle," said Dr Mosconi.
"These changes are likely driven by the fluctuations in oestrogen and progesterone levels that occur during the menstrual cycle. Our findings help us to better understand how the menstrual cycle can impact women's mental and emotional health."
The study was published in the journal JAMA Psychiatry.
