(CNN) Women expect the physical changes of pregnancy, yet having a baby also produces some changes in the brain.
Pregnancy alters the size and structure of brain regions involved in understanding the thoughts, feelings, beliefs and intentions of others, according to a study published Monday in the journal Nature Neuroscience.
Mothers with the greatest degree of overall brain change scored higher than others when tested on the strength of their maternal bonds, the researchers discovered. Many of the changes lasted two years after giving birth.
"We haven't investigated whether these changes stretch beyond this period," said Elseline Hoekzema, co-lead author of the study and a senior brain scientist at Leiden University in the Netherlands.
In rodents it is known that some of the changes in brain and behavior following a pregnancy last until old age, Hoekzema noted. In humans, it's not clear -- Hoekzema followed the participants in her own human study for two years only.
Because of the new study's short timeframe, how long lasting these changes may be in women remains unknown, said Dr. Rodney L. Wright, associate professor of clinical obstetrics & gynecology and women's health at Albert Einstein College of Medicine and Montefiore Health System.
"The significance of these changes and the duration of these changes is yet to be determined," said Wright, who was not involved in this research.
Hoekzema and Erika Barba-Müller, co-lead author and a psychologist, began the study while working together at Autonomous University of Barcelona.
Pre-conception, 25 women who became mothers for the first time and 19 of their male partners underwent high resolution MRI brain scans. After completing their pregnancies, these same participants were re-scanned. For comparison, 20 women who had never given birth and 17 of their male partners were also scanned at the same time intervals.
The new mothers showed a loss of gray matter in several brain areas associated with social cognition, a form of emotional intelligence.
While changes to the brain were clear, how to interpret them is not.
"Loss of volume does not necessarily translate to loss of function," said Hoekzema, "Sometimes less is more." She explained that the loss of gray matter could "represent a fine-tuning of synapses into more efficient neural networks."
Our teen-aged brains undergo a similar process of "synaptic pruning," explained Hoekzema. At that developmental period, weaker brain connections are eliminated, leaving a more efficient and more specialized neural network, she said. Adolescents with a more "mature" network -- meaning, less grey matter -- actually show increased brain activity in their thinned-down regions, she observed. "Reduced volume does not necessarily reflect reduced brain activity," said Hoekzema.
In fact, participants of the new study took cognitive tests during their MRI session with no significant changes seen over time. However, following their pregnancies, the mothers had fewer correct responses on the verbal word list learning task, though to an extent considered insignificant by the researchers.
Reduction in gray matter occurred in various regions of the brains of pregnant women, including the prefrontal and temporal cortex.
"These areas are involved in a number of behaviors," noted Dr. Kim Yonkers, a professor in psychiatry and obstetrics and gynecology at Yale School of Medicine who was not involved in the new study. She explained some of these regions are involved in memory, while others are implicated in depression.
Changes in these areas may help women forget the pain or difficulty of pregnancy, suggested Yonkers.
Nearly identical reductions in gray matter were seen among the mothers who used fertility treatments as compared to those mothers who became pregnant naturally.
David Van Essen, co-principal investigator of the NIH's Human Connectome Project, noted that there are other possible interpretations than gray matter volume changes. It could be an increase of myelin, he said, which could "masquerade" as gray matter volume change. Recent research suggests more myelin could speed conduction of nerve impulses in the brain.
"I think that an increase in myelin rather than a decrease in gray matter volume is a highly plausible alternative interpretation of their results," said Van Essen, noting that no matter how the results are interpreted, "there is reasonably strong evidence that 'something' is different in the brains of postpartum women."
"At this point the results are associative," said Yonkers, who noted the authors included "strong controls in the study including women who were not pregnant and men." It remains unknown whether changes last beyond two years, she said, and "we don't know what happens with multiple pregnancies."
Overall, Yonkers, who has a long-standing interest in psychiatric disorders in women, finds the study "provocative."
"Pregnancy is part of many women's healthy lifestyle and it makes sense that it would confer benefits," said Yonkers. She believes better understanding how the brain changes during pregnancy could lead to treatments for some medical conditions.
For instance, since attachment or connection was greater in women during pregnancy and postpartum, Yonkers speculated this knowledge might be applied to conditions where emotional response is poor -- such as autism.
While the current study does not provide enough evidence for such treatments, Hoekzema observed that "in rats, it is known that some of the neural changes and the effects on maternal behavior can be simulated by administration of a regimen of hormones similar to pregnancy."
According to Albert Einstein College of Medicine's Wright, the small number of participants made it impossible for the researchers to accurately say whether the brain changes they observed might have been influenced by other factors, such as breastfeeding or stressors in the home related to pregnancy.
Still, based on changes in the teen brain associated with "hormonal surges" and changes in the menopausal brain tissue that follow a significant drop in estrogen, Wright said it "is not entirely surprising that pregnancy, which is also associated with rapid changes in estrogen levels, might also be associated with changes in brain tissue."
The study is "exciting," even if it "unfortunately leaves us with many questions," Wright said. "There still is so much not yet known about the human brain."