from delancyplace.com:

In today's encore excerpt - a new and growing body of research is revealing that
there are marked and generally positive physiological changes that occur in a mother's
brain as a result of giving birth to a child. Experiments and observations on human
mothers and laboratory rats provide the evidence:
"Research suggests that motherhood enhances certain types of cognition, improves
resistance to stress and sharpens some kinds of memory. On the face of it, the
fact that the nervous system manages to transform a new mother from a self-centered
organism into an other-focused caregiver is actually quite impressive. All it takes
is for new neurons to sprout, certain brain structures to blossom in size and waves
of powerful hormones to batter the pregnant woman's physiology. The result is a
different and in some ways better brain -- or at least one capable of juggling the
challenges of everyday life while maintaining a laserlike focus on the baby.
"Of all the senses, smell -- olfaction -- plays the largest role in reproduction.
Females rely on their sense of smell from the very beginning to help them select
their mates all the way through to the weaning of their young, during which scents
act as a form of communication between mother and child. An extreme example of the
power of smell is known as the Bruce effect, a phenomenon in which certain scents
induce abortions in pregnant rodents. If a female's mate disappears after conception
and an interloper starts hanging around, the new male's smell will inhibit the production
of key hormones, causing the female's pregnancy to abort. Otherwise, chances are
high that the interloper would end up killing and eating the pups, thereby obtaining
a high-protein meal and removing a rival's genes in the bargain. ...
"During a rat's pregnancy, for example, we know that the olfactory system starts
churning out new neurons. The theory is that the extra neurons allow moms to become
more adept at processing the cues hidden in infant odors. Indeed, mothers distinguish
themselves quite obviously in how they react to smells. Whereas virgin female rats
find the odors of infants noisome, once they become pregnant, those smells attract
them. ...
"Mother rats seem to excel at tasks that require enhanced attention. Behavioral
neuroscientist Kelly Lambert of Randolph-Macon College and her colleagues have collected
other evidence of sharp-witted mothers. In 2009 they showed that when it comes to
identifying which cue among several signals food, mother rats perform best. And
recent work by Amy Au and Tommy Bilinski in our lab has begun to identify the rats'
strengthened ability to deduce the meanings of symbols. The researchers designed
experiments where a rat in an environment learns to associate, say, a triangle
or a set of wavy lines with a food reward. After being moved to a new environment,
lactating females transferred their knowledge from the old setting to the new one
better than virgin females did, again suggesting a heightened attention to detail.
"A human mother's brain undergoes a striking structural metamorphosis, too. Last
year using magnetic resonance imaging studies, neuroscientist Pilyoung Kim, now
at the National Institute of Mental Health, and her colleagues found significant
increases in gray matter in mothers' brains in the weeks and months after they give
birth. Gray matter, which got its name from the color of cell bodies, is a layer
of tissue packed with neurons. The growth the scientists saw was particularly visible
in the midbrain, parietal lobes and prefrontal cortex -- all areas involved in infant
care. The mothers with the biggest increase in gray matter volume also reported
the more positive perception of their babies.
"As the time of delivery nears, powerful hormones swing into action. Although the
most obvious players are oxytocin, which stimulates uterine contractions and milk
letdown, and prolactin, which instigates milk production, other hormones trigger
changes inside the brain, too. ... Meanwhile the hypothalamus ramps up the feelings
of pleasure a mother receives. Robert S. Bridges of the Tufts Cummings School of
Veterinary Medicine and his colleagues found different concentrations of opioid
receptors in female rats depending on whether the rodent was a virgin, pregnant
or lactating. ... The drug analogy, by the way, is not spurious. Animals may in
fact be engaging in maternal behavior simply because it feels good. Many human
mothers report a very pleasurable feeling as they breastfeed their infants. After
pups attach to a female rat's nipple, the mom receives a 'hit' of reinforcing opiate."
Author: Craig Howard Kinsley and Elizabeth Meyer
Title: "Maternal Mentality"
Publisher: Scientific American Mind
Date: July/August 2011
Pages: 25-29