Women’s brains change across the menstrual cycle, new studies find

A woman’s menstrual cycle is driven by the ebb and flow of hormones that prepare the body for pregnancy. This symphony of hormones not only transforms the reproductive organs, but, according to recent research, also reshapes the brain.

Two recent studies performed detailed brain scans of women at multiple points across the menstrual cycle, finding that the volume or thickness of certain regions change in sync with hormone levels.

The areas of the brain highlighted by both studies are those in the limbic system, a group of brain structures that govern emotions, memory and behaviour.

“These brain changes may or may not alter the way we actually act, think and feel in our everyday lives. So the important next steps for the science are to put those pieces of the puzzle together,” said Adriene Beltz, associate professor of psychology at the University of Michigan, who was not involved in the research.

Viktoriya Babenko, who conducted the research as a doctoral candidate at the University of California at Santa Barbara, and her former colleagues at the university used advanced methods in magnetic resonance imaging (MRI) to map structural changes in the whole brain during three menstrual cycle phases – ovulation, menstruation and mid-luteal.

The mid-luteal phase occurs between ovulation and menstruation, and is marked by a peak in the hormone progesterone. The participants – 30 young women with normal cycles – also had their blood drawn to precisely record their hormone levels at the time of the scans. The results, which have not yet been peer-reviewed, were posted to the pre-print database bioRxiv.

Higher concentrations of oestrogen and luteinising hormone, which triggers the release of the egg, were both associated with changes that suggest faster information transfer, as the researchers saw when imaging the white matter of the brain.

The white matter is deeper brain tissue made up of nerve fibres that relay information to and from parts of the outermost cerebral cortex, known as grey matter – which, in turn, became thicker with a rise in follicle-stimulating hormone, which prompts the growth of an ovarian follicle into a mature egg.

Follicle-stimulating hormone “was positively associated with cortical thickness, generally across the whole brain”, said Elizabeth Rizor, a doctoral candidate in dynamical neuroscience at the University of California at Santa Barbara, and one of the authors of the study. “Whereas progesterone was kind of the opposite – it was generally associated with less cortical thickness in most regions.”

The other study, published by Nature Mental Health, scanned 27 healthy participants with high-resolution MRI during

six menstrual cycle phases – menstrual, pre-ovulatory, ovulation, post-ovulatory, mid-luteal and pre-menstrual.

The researchers focused on the hippocampus and surrounding areas of the medial temporal lobe, which support a broad range of cognitive and emotional functions. They performed blood draws at each of the six time points to correlate brain changes with concentrations of oestrogen and progesterone.

Increases in oestrogen were linked to expansion of the parahippocampal cortex, a grey matter cortical region that plays a role in memory encoding and retrieval.

Elevated progesterone was tied to greater volume in the perirhinal cortex, an area that receives sensory information and is also important for memory. And high oestrogen in combination with low progesterone was associated with an enlarged region of the hippocampus, one essential for autobiographical memory.

“We know hormones drive physiological functions across the whole body. But nobody really knew how it affected the brain in humans,” said pre-print study author Emily Jacobs, associate professor of psychological and brain sciences at the University of California at Santa Barbara.

In the early 1990s, a groundbreaking experiment revealed the dramatic effects of oestrogen on the brains of female rats.

Scientists counted the number of dendritic spines – small protrusions along the branches of neurons that serve as points of contact from one cell to the next - in the hippocampus across the four- to five-day rodent menstrual cycle.

A greater density of dendritic spines translates to enhanced connectivity between neurons in the brain.

At the beginning of the cycle, when oestrogen is low, dendritic spine density is at its lowest point. Over the next several days, oestrogen levels gradually rise and peak. During this time, more spines begin to proliferate, and spine density increases by around 30%. Toward the end of the cycle, spines retract, and the process starts all over again.

The current results suggest that a similar cyclical fluctuation in neurons could be happening in humans as well.