Sleep-deprived, bleary-eyed new mothers often say their brains "turned to mush" after giving birth — "I was even more forgetful, absent-minded, and confused than I usually am," says Sierra Black at Babble. But the old beliefs about "mommy brain" may be wrong. In fact, according to a new study published in Behavioral Neuroscience, childbirth actually boosts a woman's brainpower. (Watch a Fox News discussion about the study.) Here, a quick guide to the findings:

What did the scientists discover?
Women's brains grow after childbirth. The scientists scanned the brains of 19 women a few weeks after delivery day, and again three to four months later. They showed "small but significant" growth in brain regions — including the prefrontal cortex, hypothalamus, and amygdala — that are responsible for reasoning and planning, as well as motivating mothers to take care of their babies, and feeling pleasure in motherhood. These changes could explain the concept of "maternal instinct" — the women in the study who spoke most positively about motherhood had the biggest changes in their brains.

So maternal instinct is not truly an instinct?
Maybe not. This study suggests it might really stem from physical changes in the brain. But there are clear reasons why women would be programmed this way. "From an evolutionary standpoint, a mother is faced with a really significant challenge," says Craig Kinsley, a neuroscientist at the University of Richmond. "She had to do everything she did before, plus a whole new suite of behaviors to keep her offspring alive. How females evolved in nature is to have their brains adapt in pregnancy, so that their young enhance their behaviors."

Why do many women feel less sharp after giving birth?
It may simply be that they feel like they have "mommy brain" because they have new priorities. "We are clearly showing that mothers have better memories about things related to their infants," says neuroscientist Pilyoung Kim, the study's lead author and a new mother herself. "There are a lot of things going on, and mothers might feel forgetful about things that are not related to their infants. It's just dependent on what is really important for us to remember at the time."

If new mothers' brains get bigger, do they get smarter?
Possibly. While the study only looked at brain size, not cognition, the two are closely associated. "There's certainly reason to think it might indicate that they got smarter," says Kim. "It could be [smarter] particularly in the parenting context or more generally. We don't know." And some of the changes have little to do with smarts. Some of changes in new moms' brains were in the pleasure regions, which are also altered in the brains of addicts. "You might say," says neuroscientist Elizabeth Meyer, "we're addicted to motherhood."

Sources: Discovery, Time, Parent Dish, American Psychology Association, Babble