Women's brains may change slightly every month, in sync with their menstrual cycles, a new study of one woman's brain finds.

Women experience monthly hormone fluctuations that influence the switching that takes place between their infertile and fertile days, and in a small study of just one woman, whose brain was scanned every few days throughout two menstrual cycles, the researchers found that these estrogen-level changes also affect the hippocampus — the area of the brain that is central to memories, mood, and emotions.

The researchers looked at 30 women and measured the levels of estrogen in their blood. The women also underwent MRI scans of their brains, which the researchers used to measure the volume of the different regions of each woman's brain.

As the estrogen levels rose, the hippocampus increased slightly in volume, the study showed. The researchers' measurements showed that both the grey and white matter in the brain increased as estrogen levels rose, causing the hippocampus to increase in volume.