The Menstrual Cycle Can Reshape Your Brain: New MRI Studies Reveal Rapid Hormonal Effects on Key Regions

Recent research using advanced brain imaging has shown that the menstrual cycle actively reshapes certain areas of the brain, particularly those involved in emotion, memory, behavior, and information processing. While the structural changes are clear and dramatic, scientists emphasize that these findings do not yet prove a direct connection to the emotional fluctuations or “period brain fog” many women experience.

Elma Jashim, a recent college graduate preparing to start medical school this fall, is both excited and anxious about the monthly emotional shifts she experiences during her menstrual cycle—and how they might affect her demanding academic schedule.

“For about two or three days before I begin my period, I kind of feel like, not really emotional, not particularly sad, but not particularly happy either,” Jashim says.

This mood plateau heightens her sensitivity to even small emotional triggers once menstruation begins. “If I’m at work and I make a very minor mistake, it almost sends me to the point of tears.”

What exactly happens with what some call “period brain” remains poorly understood. However, progress is being made in visualizing how hormonal fluctuations alter specific brain areas.

Previous studies in rats and other mammals had already shown that estrogen—a hormone essential for normal sexual and reproductive development in women—can change the volume of certain brain regions. Until recently, it was unknown whether this potent hormone could also reshape the structure of the adult human female brain.

Now, high-resolution MRI scans of women’s brains across multiple points in their menstrual cycle—the roughly 29-day hormonal cycle that prepares the reproductive system for possible pregnancy—have revealed that the brain undergoes dramatic remodeling in regions governing 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 the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany, who led one of the studies.

Catherine Woolley, a neurobiologist at Northwestern University in Evanston, Illinois, notes that the brain’s responsiveness to the menstrual cycle is especially significant because most women experience nearly 450 menstrual cycles over 30–40 years.

The strength of these studies lies in the fact that brain imaging and hormone measurements were taken from the same individuals at specific phases of the menstrual cycle, Woolley says.

“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

The menstrual cycle repeats every 25 to 30 days and begins with menstruation—the shedding of the uterine lining. Female sex hormone levels in the blood are lowest at the start of the cycle but rise sharply over the following weeks.

• Estrogen levels rise first, signaling the uterine lining to thicken.
• Estrogen then drops to trigger ovulation, releasing an egg from the ovary at the cycle’s midpoint.
• Progesterone and estrogen levels rise again for about seven days to prepare the uterus for possible fertilization.
• If no pregnancy occurs, both hormone levels fall, triggering menstruation.

While the menstrual cycle involves a pronounced seesaw of estrogen and progesterone, other hormones such as testosterone and cortisol also follow daily rhythms—rising before dawn and falling in the evening—in both men and women.

Estrogen Stimulates Cognitive Brain Regions

The brain is made up of neurons—dense, tree-like cells. The outer layer, called gray matter, contains the neuron cell bodies and their short branches (dendrites), which have leaf-like protrusions called spines. The axons, or roots of the neurons, bundle together in the deeper white matter, which transmits information between different gray matter regions.

Gray matter regulates emotion, learning, and memory, while white matter handles communication across the brain.

Nearly three decades ago, in 1990, Woolley serendipitously discovered that estrogen regulates the density of dendritic spines in the hippocampus of rat brains—a finding that initially faced skepticism because estrogens were then considered solely reproductive hormones, not influencers of cognitive regions like the hippocampus.

The hippocampus, a small, curved structure deep behind the ears, is rich in sex hormone receptors. It is also the most volume-responsive region in the adult human brain: learning new skills (such as juggling or studying maps for a London taxi license) can enlarge it, while shrinkage can be an early sign of dementia, particularly Alzheimer’s.

Since Woolley’s discovery, researchers have learned that menopause reduces gray matter volume in certain brain areas. However, previous studies were limited to single snapshots of the brain. Scientists wanted to know whether the adult brain changes during the monthly hormonal fluctuations of the menstrual cycle.

New Studies Track Brain Changes in Real Time

Emily Jacobs’ team at UC Santa Barbara asked: “Can we get really precise? Can we scan one person’s brain 30, 50, or 100 times?” In 2020, one scientist in her group underwent daily brain scans for a full month.

“She was like the Marie Curie of neuroscience,” Jacobs says. From 30 scans of this single woman, the team found that sex hormones reshaped the hippocampus and reorganized brain connections. But the speed of these changes during the menstrual cycle remained unclear.

To answer this, two independent teams in Leipzig and Santa Barbara scanned the brains of more than 50 healthy women at multiple points during their menstrual cycles.

Sacher’s Leipzig team (published in Nature Mental Health) used ultrasound to precisely time ovulation in 27 volunteers. They collected blood samples at six key points linked to hormone levels and ovulation, then scanned the women’s brains at those same points using ultrahigh-field MRI—capable of resolutions previously only possible in postmortem brain slices.

Despite the hippocampus being a small structure, they observed a choreographed series of changes: the outer layer thickened and gray matter expanded with rising estrogen and falling progesterone, while the memory-related layer expanded when progesterone rose.

The Santa Barbara team scanned 30 volunteers during ovulation, menstruation, and the interval between. They found fluctuations not only in gray matter thickness but also in the structural properties of white matter, suggesting more efficient information transfer across brain regions around ovulation.

“We applied kind of a ruler [to gray matter] and saw it change in concert with the hormone fluctuations,” says Elizabeth Rizor, co-leader of the study with Viktoriya Babenko, both neuroscientists at UC Santa Barbara.

Babenko added: “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.”

Important Limitations

While the structural changes are widespread and rapid, scientists caution that:

• Bigger or thicker brain regions do not automatically mean better function.
• The studies included only healthy women who did not report significant emotional or cognitive symptoms during their cycles.
• No direct link has yet been established between these volume/thickness changes and common menstrual symptoms such as mood swings, emotional sensitivity, or “period brain fog.”

Why This Research Matters

Most neuroscience research historically focused on men or postmenopausal women, largely overlooking monthly hormonal dynamics in cycling women. These studies highlight the urgent need for more female-specific brain research, especially since hormonal fluctuations continue across decades of reproductive life.

As Emily Jacobs notes, the findings show “how potent these hormones are for shaping not just brain morphology but also the functional architecture.”