Pregnancy is known to cause permanent physical changes, including hormonal changes and transformations of reproductive and abdominal tissues. But little is known about the effects of pregnancy on the brain, although some previous studies suggested changes in regions related to emotional regulation.
But a recent study in Natural Neuroscience presents striking findings that pregnancy induces substantial changes in brain structure. This is real proof that there is a definite ‘pregnancy brain’. But there is no evidence that these changes cause the forgetfulness popularly attributed to pregnant women.
For this study, researchers looked at new mothers and used women who had never had children as a control group. Researchers took MRI scans of the mothers both before and after their pregnancy, allowing each woman’s pre-pregnancy brain scan to serve as the basis for understanding her post-pregnancy scan. The brains of the expectant mothers and the controls showed no differences at baseline.
After pregnancy, however, a stark difference in brain structure emerged between the two groups: Women who were pregnant showed a significant reduction in gray matter that persisted up to two years into pregnancy. These findings were so important that just by looking at the brain scans, the researchers could tell which women were pregnant and which were not.
In the brain, gray matter is distinguished from white matter because it is densely populated with neuron cell bodies, which cause the gray color. It differs from white matter, which consists largely of the thread-like axons that nerves use to communicate with each other.
The effects of reduced gray matter are unclear. While some previous studies have shown that people with more gray matter have better memories and greater happiness, the reduction in gray matter isn’t necessarily a bad thing. This reduction may be part of a natural process known as “pruning,” in which the brain increases its efficiency by removing neurons or neuronal networks it doesn’t need. Extensive pruning normally takes place during adolescence, another period of significant physiological change. The pregnant brain may be undergoing a similar process, though researchers need more information to understand what’s going on.
The study showed that these decreases after pregnancy occurred in all “association areas” of the brain, which are known to play key roles in social processes. In fact, the pattern of the affected areas resembled the “theory of the mind” neural network. The theory-of-mind network in the brain is believed to allow people to understand what other people might be thinking or feeling. This network also processes internal emotional states.
The researchers speculate that the reduction in gray matter may occur because the female brain experiences a specialization of the neural network that supports social interaction during pregnancy. This specialization helps prepare them to interpret the needs of their soon-to-be children. Supporting this idea, the authors found that the gray matter reduction is located in several brain regions that had the strongest activation response in infants born to women when the mothers were subjected to a postpartum fMRI task.
To look at any effects that may coincide with these brain changes, the researchers tested the cognitive performance of the new mothers and childless controls both before and after pregnancy. They found no significant differences between the two groups either before or after pregnancy. This suggests that the morphological changes that occur in women’s brains as a result of pregnancy have no adverse cognitive effects.
To confirm that these results were due to the pregnancy and not brain changes caused by the parenting task, the researchers also looked at the brains of new fathers before and after their partner’s pregnancy, comparing them to a group of childless men. No similar brain changes occurred in the men. This indicates that the reduction in gray matter volume experienced by the women is related to the pregnancy process, not parenting.
These findings strongly suggest that the informal “pregnancy brain” and “mommy brain” stages may have a basis in the physical structure of the brain. However, based on this current study, there is no reason to suspect that these changes alter cognitive abilities. It is possible that pregnancy-induced changes in gray matter are an adaptive process that helps women transition into motherhood, but more research is needed to truly understand the purpose and consequences of these changes.
Natural Neuroscience2016. DOI: 10.1038/nn.4458 (About DOIs).