How the Brain Changes During Pregnancy

Science in general, and neuroscience in particular, have long focused on male individuals as the prototypical organism.

As recently as 2014, a meta-analysis of thousands of neuroscientific studies in rats found that 58 percent used only male animals, compared to 9 percent that used only females; a further 14 percent didn’t even report the animals’ sex.

Thankfully, this is changing. Indeed, it has always struck me as odd that we would focus on males when they are totally incapable of performing the most critical of biological acts, namely being pregnant and giving birth. A 2024 study makes a tremendous step toward understanding the neuroscientific implications of precisely this feature of our biology.

Using non-invasive brain imaging, Elizabeth Chrastil of UC Irvine and Emily G. Jacobs of UC Santa Barbara and their colleagues investigated the structural changes to the brain of one woman from before pregnancy through to postpartum, totaling more than two years of study.

(It is worth pausing at the outset to acknowledge the depth of commitment that this kind of study required from both the researchers and the subject. Any number of life changes and decisions for both parties would surely need to have been weighed against the trajectory of the study. This level of dedication is also highly relevant to recent cuts to federal science funding, especially sex-specific science.

The great enemy of progress is uncertainty, and the opportunity costs to future breakthroughs are likely to be significant. I, for one, am thankful that Chrastil, Jacobs, and colleagues began their study when they did.)

The subject of the study underwent in vitro fertilization, so pregnancy status was known throughout the study. Her brain was imaged in 26 separate scans using magnetic resonance imaging.

Anyone who has ever had an MRI scan might be thinking now that this would be pretty unpleasant, and they would be right. Scanning is an hours-long process of immobility and deafening noise that often triggers loneliness, boredom, and claustrophobia. Now imagine doing this 26 times—while pregnant. I have only done it once, not pregnant, and that was quite enough for me.

The researchers used anatomical and diffusion spectrum imaging to study brain structure. Examination of anatomical structure showed a dramatic loss of gray matter volume over the course of pregnancy. Gray matter contains most of the cellular machinery of neurons, including nuclei, dendrites, and most of the neuron’s organelles, as well as cells that support neurons (glia). The reduction in volume was observed in almost all parts of the brain, including nearly all of the cortex, as well as subcortical structures like the thalamus, hypothalamus, and hippocampus.

What cellular parts of the gray matter were being lost was not determined—most likely, it was not simply due to mass cell death but rather to small changes within billions of neurons that together altered gray matter volume in major ways.

In contrast, the subject’s white matter—long axons connecting brain regions within and across brain hemispheres—showed increases. The researchers applied a technique called quantitative anisotropy to the imaging data. This analysis measures asymmetries in the flow of water molecules along axons to gauge the length and thickness of the white matter tracts.

Though this method does not give as clear an indication of the absolute amount of brain volume change, it did show strong relationships over time. White matter connectivity strength followed pregnancy hormone levels in many parts of the brain.

What Can This Study Tell Us About Pregnancy and the Brain?

The results of this study are so novel and unique that it is not yet clear what to make of them. On top of this, we still mostly lack an understanding of the importance of changes in the gross structure of the brain in people who aren’t pregnant, as well as in terms of sex. Brain imaging studies of this kind involve a generous amount of inference based on ambiguous data, especially for white matter measures. The tradeoff for non-invasiveness is a lack of precision.

Precise and comprehensive invasive studies of white matter connectivity in mice (Oh et al., 2014) and monkeys (Markov et al., 2014) are composed of data combined from a few dozen individuals. Neither of these landmark studies factored in the role of sex: Oh et al. (2014) only studied male mice, and Markov et al. (2014) didn’t report the sex of the 28 monkeys they studied. (Somehow, this fact had previously escaped my notice despite often employing both of these valuable datasets in my own research.)

If nothing else, the work of Chrastil, Jacobs, and colleagues points the way toward a future of discovery through highly ambitious and mold-breaking approaches to brain science. Let’s hope the scientific community—as well as funding agencies and society at large—recognize this opportunity and redouble its support for it.