Jeffrey Borenstein, M.D. and Brain & Behavior Staff
Pregnancy is a time of profound changes in the female body, including the endocrine system—the system that secretes and regulates hormones. Changes in female sex hormones such as estrogen orchestrate numerous adaptations throughout the body, including the brain. Yet there is notably little data reflecting the impact of the reproductive process on the female human brain.
To address this, 2017 BBRF Young Investigator Elseline Hoekzema, Ph.D., of the University Medical Center of Amsterdam and the University of Leiden, the Netherlands, led a team that recruited 80 women of childbearing age, half of whom became pregnant over the period of the study, and a matching group who were not pregnant during that time.
Dr. Hoekzema and colleagues used four technologies to examine brain structure and function in these women, at four time points. Tests were performed on all of the women at baseline, which was prior to conception for those women who became pregnant; and then at times corresponding with late pregnancy, post-childbirth, and the late postpartum period among those participants who became pregnant.
These tests enabled the team to assess brain structure and function across the four time periods and supported a series of important findings about how pregnancy affects resting-state brain activity, the structure of the brain’s white and grey matter, and levels of neural metabolites—molecules generated by or as a result of metabolic processes occurring in brain cells.
Broadly speaking, the team, reporting in Nature Communications, said their data revealed “pronounced and selective structural and functional” changes in brain plasticity, “which may confer adaptive advantages” affecting the mother’s behavior in the forging of bonds with a new child. Plasticity refers to changes in the strength of connections between neurons and is a key factor in how well the brain functions, for example, in learning, memory, and the ability to respond and adapt to changing bodily or environmental conditions. Deficiencies in neuroplasticity have also been linked with depression and other psychiatric illnesses.
The tests conducted at baseline showed that there were no pre-existing differences in the volume of the brain’s grey matter among women in the two groups. Grey matter corresponds, roughly, with portions of the brain composed of neuronal cell bodies; white matter refers to the structures such as axons that connect neurons into complex networks.
Based on data from structural MRI scans that each woman received over the course of the study, the team was able to confirm its own previous finding indicating that women experiencing pregnancy and the postpartum period have reductions in the volume of their grey matter described as “highly significant” with “very large effects.” The new findings confirmed not only the magnitude of the previously observed reductions but also the locations in the brain where they appear to be most prominent.
Resting-state fMRI scans showed that despite reductions in grey matter among women who were pregnant or in the postpartum phase (compared with the controls who were not pregnant at these time points) there was a notable “increase in functional connectivity” that was evident in the brain’s default mode network (DMN). The DMN regulates brain activity at moments when an individual is not focusing on the external world. Specifically, the team found that reproductive processes enhanced the DMN’s “temporal coherence.”
A different brain-scanning technology called diffusion tensor imaging revealed that there was no significant change in white matter structure among the pregnant/postpartum women when compared with those in the control group. Similarly, measurements of neural metabolite concentrations revealed no strong changes.
Perhaps the study’s most important finding was that pregnancy-related neural changes were likely associated with the stimulation of behavioral and bodily adaptations that new mothers normally make to prepare for motherhood. For instance, pregnancy-related neural changes were associated with changes in the mothers’ physiological responses to infants, to nesting behaviors, and to bonding with newborns in the postpartum period.
While the observed changes in brain structure were maintained, the increases in DMN coherence gradually reverted back to pre-pregnancy levels during the postpartum period. The total duration of breastfeeding positively correlated with the gains in DMN coherence, suggesting to the team that “prolonged breastfeeding may stimulate a prolonged maintenance of pregnancy-related neural changes.”
During the perinatal period, changes in the DMN, which plays a key role in self-perception, may even “underlie transformations in the neural representation of the self when becoming a mother,” the researchers speculated. Sex hormones and especially estrogen appear to contribute to these adaptive brain changes. Broadly speaking, they said, “our findings suggest pregnancy-related neuroplasticity plays a role in psychological and physiological gestational maternal processes that help a woman to prepare for the arrival of her baby” and to “the establishment of the mother-infant dyad.