The present study investigated changes in the structure and plasticity of the maternal brain during pregnancy. The authors have explored changes in the organization of the neural network, concentrations of neuronal metabolites and the structure of white or grey matter in the human brain during pregnancy.
About the changes in the maternal brain during pregnancy
The relationship between the pregnant woman and her developing fetus is perhaps the most profound, but enigmatic of all human relationships. Prenatal synchrony between mother and fetus helps to ensure the survival of the offspring. A converging body of evidence has shown that reproduction is associated with a unique brain plasticity in various animal species.
Pioneering research by Bianki et al (1999) demonstrated a shift in the magnitude of cerebral lateralization in pregnant cats. They found changes in transcallosal inter-hemispheric connections in different cortical areas, and, also, that the focus of maximum cortical activity moved in the latero-caudal direction. According to the authors, these findings reflect changes in the functional asymmetry of the maternal brain and the reorganization of pregnancy-specific cortical networks.
Human neuroimaging studies have demonstrated numerous changes in the brain at the physiological, cellular and molecular levels during pregnancy and lactation. Changes in the physiology and plasticity of the maternal brain during pregnancy prepare the female for the challenges of motherhood and contribute to the survival of her offspring. Various brain regions, including the substantia nigra, amygdala, thalamus, parietal cortex, and prefrontal cortex are activated in response to the fetal-related stimuli.
Some earlier studies have shown that changes in the structure of the animal maternal brain associated with reproduction persist throughout life, whereas changes in the structure of the human maternal brain last at least two years after the birth.
About the study
In this comprehensive prospective cohort study, the research team used magnetic resonance imaging (MRI), diffusion-weighted imaging, 1H-nuclear magnetic resonance spectroscopy and resting state functional MRI. They have examined whether pregnancy is associated with changes in the organization of the neural network, the concentrations of neuronal metabolites and the structure of white or grey matter.
In addition, they analyzed hormonal changes in biological samples taken every four weeks of pregnancy in combination with experiential factors such as sleep and stress. The research team also studied the temporal coherence within and between neural networks by comparing the prepregnancy and postpartum brain scans of pregnant women with those of nulliparous women. The women were followed from the preconception to the late postpartum period.
The results showed a highly significant reduction in the volume of grey matter in pregnant women compared to nulliparous women. These changes primarily affected the anterior and posterior cortical midline and specific sections of the bilateral lateral prefrontal and temporal cortex. The correlation between the changes in grey matter volume and the marker of glial cells (myoinositol) indicates that the observed changes at least partially reflect the changes in glial cells.
In contrast to the highly pronounced changes in the structure of grey matter, the white matter diffusion metrics or volume did not show any significant changes in pregnant women compared to nulliparous women. The authors suggested that the structure of white matter in the maternal brain remains relatively stable during this period.
The results also showed that pregnancy causes selective and robust changes in neural architecture and neural network organization, which were mainly pronounced in the Default Mode Network (DMN). DMN is a group of highly interconnected brain regions that is most active in the absence of a specific task, reflecting the baseline activity of the maternal brain. DMN is involved in autobiographical memory, self-perception, and social processes such as social cognition, social evaluation and empathy. Data from the resting state functional MRI, used to investigate the temporal coherence within and between neural networks, showed a selective increase in DMN coherence in pregnant women compared to the control group.
This included increased DMN coherence in the cuneus which plays a key role in visual processing and the integration of visual information with processes such as working memory, attention and reward expectation. The authors suggested that anatomical and resting state MRI findings indicate that pregnancy is associated with structural and functional plasticity within the DMN. This suggests that pregnancy alters the baseline state of the maternal brain.
Observed neural changes were correlated with the levels of gonadal hormones, primarily with levels of estradiol during the third trimester. Gonadal steroids affect brain development and differentiation, and influence neuronal functions. The researchers speculated that marked changes in the structure of grey matter compared to insignificant changes in the white matter diffusion metrics or volume could reflect a particularly strong sensitivity of grey matter to fluctuations in gonadal hormones.
Finally, the researchers investigated if the observed neural changes were persistent during the postpartum period. The findings showed that the changes in the structure and plasticity of the maternal brain were partially reversed during the postpartum period. The results showed positive correlation between duration of breastfeeding and the degree to which the increased DMN coherence had reverted during the postpartum period. This suggests that prolonged breastfeeding can stimulate a prolonged maintenance of the neural changes associated with pregnancy.
The authors concluded that these results revealed marked and selective changes in the structural and functional plasticity of the maternal brain, during pregnancy, which are of crucial importance for mother-infant dyad.
This article was published in the scientific journal Nature Communication. Hoekzema E. et al. Mapping the effects of pregnancy on resting state brain activity, white matter microstructure, neural metabolite concentrations and grey matter architecture. Nat Commun 13, 6931 (2022). (Open Access) https://doi.org/10.1038/s41467-022-33884-8