FRIDAY, Oct. 22 (HealthDay News) -- The brains of new mothers actually get bigger within months of giving birth, according to new research.

The researchers, most from the Yale University School of Medicine, said that the growth was likely fueled by changes in levels of certain hormones just after birth. They reported finding expansion in areas of the brain associated with behavior and motivation.

The researchers also found that mothers who were most enthusiastic about their babies had more growth in key parts of the mid-brain -- areas linked to maternal motivation, rewards and emotion processing -- than did mothers who were more reserved about their infants.

The findings, from a small study reported in the October issue of Behavioral Neuroscience, suggest that a new mother's desire to look after her baby may be driven less by instinct and more by active brain building, according to two neuroscientists whose commentary on the study was also published in the journal.

Led by neuroscientist Pilyoung Kim, the study compared MRI images of 19 women taken two to three weeks and three to four months after they gave birth at Yale-New Haven Hospital, in Connecticut. The women averaged about 33 years old, all were breast-feeding, nearly half had other children and none had postpartum depression.

The brain scans revealed small but significant increases in gray matter volume in various parts of the brain, including areas associated with maternal motivation (hypothalamus), reward and emotion processing (substantia nigra and amygdala), sensory integration (parietal lobe) and reasoning and judgment (prefrontal cortex).

In adults, gray matter generally does not change size over a few months without significant learning, brain injury or illness, or a major shift in the environment, according to background information in a news release from the American Psychological Association.

Hormonal changes that occur immediately after birth, including increases in estrogen, oxytocin and prolactin, may make new mothers' brains more susceptible to reshaping in response to their infant, according to the researchers.

Mothers who suffer postpartum depression may experience reductions, instead of growth, in these same brain areas, the researchers suggested. They added that further research into what occurs in the brains of at-risk mothers could lead to new treatments.

More information

The Nemours Foundation has more about bonding with babies.