The menstrual cycle can reshape your brain
Studies show the menstrual cycle changes certain brain regions. But they don’t reveal whether that’s connected to emotional fluctuations.
Elma Jashim, a recent college graduate is looking forward to beginning medical school in the fall. But she is also dreading the monthly emotional roller coaster that occurs with her menstrual cycle and the havoc it could wreak with her busy academic schedule.
“For about two, three days before I begin my period, I kind of feel like, not really emotional, not particularly sad, but not particularly happy either,” says Jashim.
This mood plateau heightens Jashim’s sensitivity to even small emotional stimuli when her menstruation begins. “If I’m at work and I made a very minor mistake, it almost sends me to the point of tears.”
What exactly happens with what some call “period brain” isn’t well understood. But progress is being made in visualizing hormonal effects in altering some brain areas.
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Previous studies in rats and other mammals had already shown that the volume of specific brain regions can change in response to estrogen—a hormone required for normal sexual and reproductive development in women. But whether this potent hormone could alter the structure of the human female brain was unknown.
Now recent MRI scans of the brains of women show the effects of the menstrual cycle—the 29-day period when hormones ebb and flow that prepares their reproductive organs for a possible pregnancy—dramatically reshapes regions of the brain that govern emotions, memory, behavior, and the efficiency of information transfer.
“It’s amazing to see that the adult brain can change superfast,” says Julia Sacher, a psychiatrist and neuroscientist at Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany, who led one of the studies.
That the brain changes throughout the menstrual cycle is especially noteworthy because most women experience almost 450 menstrual cycles over 30-40 years, says Catherine Woolley, a neurobiologist at Northwestern University in Evanston, Illinois.
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The strengths of these studies are that brain imaging and hormone measurements were done in the same individuals, across specific phases of the menstrual cycle, says Woolley.
“Through these studies, we now have this picture emerging of how potent these hormones are for shaping, not just brain morphology but also the functional architecture,” says Emily Jacobs, a neuroscientist at the University of California, Santa Barbara.
How hormones drive the menstrual cycle
A menstrual cycle repeats every 25 to 30 days and begins with a “period” or the shedding of the lining of the uterus. The female sex hormone levels in the blood are lowest at the beginning of the cycle but then rise steeply over the next few weeks.
First, levels of estrogen rise, signaling the lining of the uterus to grow. Then estrogen levels fall to release an egg from the ovary marking the midpoint of the menstrual cycle.
After that, levels of progesterone and estrogen hormones increase again for about seven days to prepare the lining of the uterus for the possible fertilization of the egg. If a pregnancy does not occur, both estrogen and progesterone levels fall back initiating the period bleeding.
While the menstrual cycle results from a pronounced seesaw of the hormone levels, other hormones such as testosterone and cortisol also cycle; rising before dawn and falling in the evening. These daily rhythms occurs in both sexes.
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Estrogen stimulates cognitive brain regions
The brain’s structure consists of a dense mass of cells called neurons, each of which looks like a miniature tree. The gray matter, the outer layer of brain tissue, contains the neurons and their short branches called dendrites. Dendrites have leaf-like protrusions on them called spines. The roots, or the axons of the neurons pack together in the white matter of the brain.
While gray matter regulates emotion, learning, and memory; white matter deeper in the brain tissue exchanges information and connects different regions of gray matter.
The parts of brain that respond to the female sex hormones were first discovered almost three decades back. In 1990, Woolley serendipitously discovered that estrogen regulates the density of dendritic spines in the hippocampus of rat brains.
“This was a very surprising result and produced considerable skepticism in the field,” recalls Woolley. “At that time, estrogens were considered to be solely reproductive hormones and not to affect cognitive brain regions like the hippocampus.”
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The hippocampus—the cognitive center of the brain that contains both gray and white matter—is a small, curved structure buried deep in the brain behind the ears, in a region densely populated with sex hormone receptors.
The hippocampus is also the region of an adult human brain that is most responsive to change in volume. Developing new skills, such as learning to juggle in old age, or studying maps to pass a London taxicab driving license exam makes the hippocampus bigger. On the other hand, a shrinking hippocampus can be an early sign of dementia, particularly in Alzheimer’s disease.
Since Woolley’s groundbreaking discovery, scientists have learned that menopause decreases volume of gray matter in some parts of the brain. However, research has been limited to just getting a snapshot of the brains of volunteers at a single time point. Scientists wanted to know if adult human brains change during the monthly rise and fall of sex hormones.
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“Can we get really precise? Can we take one person and measure their brain 30, 50, or 100 times?” wondered Jacobs. This prompted one scientist in Jacob’s group to get her own brain scanned every 24 hours for a whole month in 2020.
“She was like the Marie Curie of neuroscience,” says Jacobs. From the 30 scans of this one woman’s brain, Jacobs’ team found that sex hormones reshaped the hippocampus and reorganized the brain’s connections. However, it wasn’t clear how rapidly the waves of hormones during the menstruation cycle could do this.
To address the question, the scientists in Leipzig and Santa Barbara have now independently scanned the brains of more than 50 women during multiple points during their menstrual cycles for two unrelated studies.
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Hormonal changes affect thickness of brain regions
In one study, published in the journal Nature Mental Health, Sacher’s team used ultrasound to identify the precise time of ovulation for 27 female volunteers.
This enabled them to collect blood samples from volunteers at six precise points during their menstrual cycle that were linked to ovulation and hormone levels in the blood. Then they scanned the brains of these 27 women at six specific time points using ultrahigh-field MRI.
By using this more powerful MRI than commonly employed in clinics, Sacher’s team could take images of the live brain with a resolution so high it was previously only possible by directly slicing the brain during a postmortem.
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Even though it is a very small structure, the Sacher team was able to observe a choreographed series of changes in different regions of the hippocampus as it remodeled in step with the menstrual cycle.
The outer layer of the hippocampus got thicker, and the gray matter expanded with rising estrogen levels and falling progesterone. But when progesterone levels rose, the layer involved in memory expanded.
Other research, not yet peer-reviewed, scanned the brains of 30 volunteers during ovulation, menstruation, and the duration between the two. This study found that not just the thickness of the gray matter, but also the structural properties of the white matter, fluctuated under the hormones’ cue.
“We applied kind of a ruler [to gray matter] and saw it change in concert with the hormone fluctuations,” says Elizabeth Rizor, who co-led this study with Viktoriya Babenko, both neuroscientists at the University of California Santa Barbara.
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The study suggests that effects on the brain’s white matter due to hormonal fluctuation leading up to ovulation could trigger more efficient information transfer across different parts of the brain.
“These changes are very widespread, not just in the gray matter, but also in the areas of the brain that are responsible for coordinating across regions and across white matter highways,” says Babenko.
However, the changes observed in the volume or thickness of the regions of the brain in these studies have not yet been associated with specific brain functions.
While the studies show that certain areas of the brain can remodel themselves in concert with oscillations of hormones during the menstrual cycle, scientists caution these studies do not mean that memory or cognition is affected.
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“We cannot say that bigger is better for particular brain functions or processes,” says Woolley.
The studies also don’t reveal whether the volume changes are connected to the myriad emotional and cognitive function symptoms women experience during their period. In fact, these studies included only healthy women who did not report any of these symptoms.
What these studies highlight is an urgency for more research to study women’s unique neuroscience needs, says Jacobs.
