Motherhood changes you forever—that’s no secret. The cocktails of hormones that flood your body during pregnancy bring with them a range of physiological and behavioral changes in preparation for the tiny human growing inside you. But some of these changes never go away, including those in your brain.
The changes in brain structure that accompany pregnancy have been known about for some time. But exactly how this neuronal rewiring is orchestrated has, until recently, remained a mystery. Now, researchers from the Francis Crick Institute in the U.K. have shone a light on the molecular underpinnings of this cerebral circuitry in a new study published in the journal Science on October 5.
“We found that the pregnancy hormones estradiol and progesterone structurally remodel specific neurons in the brain by binding to receptors with the ability to directly switch genes on and off,” Johannes Kohl, group leader of the State-Dependent Neural Processing Laboratory at the Francis Crick Institute and one of the study’s co-authors, told Newsweek.
To study these effects, the team analyzed how behavior of female mice changes throughout pregnancy compared to virgin mice. They then identified a group of specialized nerve cells in the brain’s coordination center, the hypothalamus, which have previously been implicated in parenting behavior.
Within these nerve cells, they found high concentrations of receptor molecules that bind to the reproductive hormones estrogen and progesterone. Removing them from the cells appeared to block normal parenting behavior in the pregnant mice but had no effect on their virgin counterparts.
Next, they looked more closely at how the binding of these hormones to their receptors affected brain activity in the maternal mice. For estrogen, the observed effects were only temporary, reducing the baseline activity of these nerve cells while also making them more excitable. But for progesterone, the effects involved permanent remodeling of the animals’ brains.
“In this particular case, progesterone binding to its receptor results in the formation of so-called dendritic spines—tiny protrusions on the information-receiving parts of neurons,” Kohl said. “These spines in turn recruit new inputs onto these neurons, thereby changing how they are integrated into parenting networks in the brain.”
In other words, progesterone indirectly creates the formation of new sites of communication between neurons. “Intriguingly, these changes are long-lasting, and pregnancy seems to permanently remodel the brain,” Kohl said. “The way these remodeled neurons are integrated in the rest of the parenting network in the brain is permanently changed by pregnancy.”
While these results were only seen in mice, this hormone-orchestrated remodeling fits nicely with previous observations made in humans. “Several studies in recent years have shown that pregnancy leads to volume changes in the human brain that are large enough to be detected using MRI scanners,” Kohl said. “But it remains unclear whether hormones are behind those changes, too, and whether these larger-scale changes are due to the microscopic changes that we have described.”
These hormonal brain changes may also contribute to feelings of “baby brain” during pregnancy—the forgetfulness, fatigue and mental fog experienced by many expectant mothers. “In mice—and potentially humans, but this still needs to be addressed—these changes very likely contribute to aspects of ‘baby brain’, like being more sensitive to infant stimuli,” Kohl said. “However, changes in other parts of the brain might in turn be behind other commonly reported changes such as forgetfulness.”
Things can also go wrong during this period of remodeling, which could potentially contribute to pregnancy-associated mood disorders. “While pregnancy hormones create a window of brain remodeling, things can go wrong during this time, which can result in, or contribute to, common disorders such as postpartum depression and anxiety,” Kohl said.
However, it is difficult to determine how important these hormonal changes are in the onset of parenting behavior in humans based on mouse studies alone. “I think it’s important to note that parenting in humans is very complex, and that in addition to hormones, many other social and environmental factors are very important,” Kohl said. “It is quite clear that you can be an excellent parent without ever experiencing any of the hormonal changes of pregnancy, and there are many possible forms of parenting, and constellations of caregivers.
“In mice, these hormonal changes, and their effects on specific neurons in the brain, are absolutely necessary for being a good parent. But even mice can become better at parenting without any hormonal changes, for example by repeated exposure to infants, or by observing mothers taking care of their young.”