With her second child growing larger by the day, Liz is experiencing the tyranny of her pregnancy. Her belly seems impossibly huge to her. Easy sleep is a distant memory now that she must contend with tens of pounds of extra girth. With fiery heartburn following every meal, she feels as if she is subsisting on a diet of small volcanoes.

But Liz is not just any late-term mother-to-be. She is also a neuroscientist studying the changes that occur in a mother's brain—in fact, she co-authored this article. Although it will not relieve her indigestion, she derives some comfort from a new and growing body of research that is revealing the marked and generally positive alterations that accrue to a mother's brain.

Because the maternal brain emerges gradually, the construction site it becomes in the interim can cause some problems for its owner. Some mothers complain of fuzzy-headedness, and certain data even show minor brain shrinkage during pregnancy. But the compensations are great. 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.

A Sensory Trigger
A baby does what he can to attract and hold his mother's attention. A young son's distinctive cry, his unique scent and the way he curls his fingers around his mother's are just a handful of the sensations that shower down on her highly sensitized nervous system. The infant creates a rich environment that stimulates the mother, pushing her brain into a higher gear.

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. In a kind of “Sophie's choice” for rodents, the female is basically making a cold calculation—better to lose the young as embryos than as pups.

Because of our limited ability to peer into human brains, rodents help us approximate the changes that are taking place inside mothers such as Liz. What we have seen so far is that the mammalian brain possesses a dramatic ability to shape-shift when life demands it. 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. Human mothers also demonstrate these effects, as psychologist Alison Fleming of the University of Toronto Mississauga and her colleagues reported. They found that mothers are much more likely to rate their infants' odors as pleasant, as compared with nonmothers.

To transform women's perceptions of smells, the olfactory system may rely on a region known as the medial amygdala, suggests neurobiologist Michael Numan of Boston College and his colleagues. This brain area could be acting as a hub for the olfactory system, with information arriving here to be processed for emotional content. The olfactory tweaks may aid in solidifying the mother-child bond by making babies' odors alluring. Before she had her first child, Liz had avoided the smells of children, even those to whom she was related. But with the birth of her son, she discovered she had no problem stuffing her nose into his diaper to determine if he needed a change.

Caution and Courage
If Liz devoted all her attention to her infant, however, both mother and child would perish. A mother rat that stays safely in the nest with its offspring also dooms them to death from hunger and thirst. Mothers of both species must find ways to resolve the competing demands on their time. In other words, women are not the only members of the animal kingdom who find themselves juggling the duties of a working mom.

To allow a rat mother to toggle between caring for its young and heading out to find food, an area of the midbrain called the periaqueductal gray (PAG) acts as a circuit breaker. In 2010 researchers at the University of São Paulo proposed that the PAG weighs the balance between eating and acting maternally by evaluating input from the brain's limbic system, a set of structures that governs survival-type behaviors. No exact parallel to the PAG's toggle function in rats has been identified in humans yet, but much has been made of a mother's superhuman ability to multitask, perhaps reflecting a similar adaptation.

When a mother ventures into the world, she puts her vulnerable baby at risk. But she may be more attuned to potential threats, perhaps even exaggerating them, suggests research at the Health Sciences Federal University of Porto Alegre in Brazil. Researchers there have shown significant alterations in the architecture of dendrites in the medial nucleus of the amygdala, which in addition to its important role in the olfactory system also controls defensiveness and avoidance behavior. Indeed, when Liz shops she scans the stores for risks to her baby, avoiding the creepy guy by the magazines or the sketchy teens by the vending machines.

Although overall Liz is more cautious, she is also probably much bolder in the face of a threat than she was before becoming a mother. Psychologist Jennifer Wartella, now at Virginia Commonwealth University, has found that, compared with virgins, mother rats exposed to a stressful open-field maze were less likely to freeze in place, explored more readily and appeared to experience less fear (that is, Wartella saw fewer switched-on neurons in the amygdala). With its fear response in check, a rat mom may be able to forage more efficiently and return to its nest and vulnerable offspring more quickly.

Helping a mother navigate the world is her improved ability to decipher the clues in the environment. Recently our student Kelly Rafferty and her colleagues at our lab have been investigating a mother's ability to plan ahead. They allowed mother and virgin rats to forage in an unfamiliar maze that contained water. The rats were then returned to their home cages, some of which contained a water bottle and some of which did not. Subsequently they were moved back to the maze containing water. The mother rats assigned to the waterless home cage spent more time near the maze's water sources and drank more water, as compared with both mothers with full access to water and virgin females. After accounting for potential differences in the rats' thirst, the neuroscientists concluded that the mothers appear to anticipate a future environment and plan for it.

As the previous experiments demonstrated, 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 work by Amy Au and Tommy Bilinski, then working 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. In 2010 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 unmyelinated axons, 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.

Maternal Morphine
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. Neuroanatomists at the Victor Segalen Bordeaux 2 University in France have observed a dramatic structural remodeling of the hypothalamus, a brain region that acts as a major regulator of the hormones associated with basic emotional behaviors such as fighting and sex. Neurons in a part of the hypothalamus known as the medial preoptic area, or mPOA, grow bigger and become more active. Indeed, lesions of the mPOA can eliminate maternal behavior.

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. But the phenomenon fades with experience. Females that go through several pregnancies show a decline in sensitivity to their own opioids, much like addicts who require ever greater doses of a drug to get high.

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. But the rat's body imposes a natural limit. As the pups continue to suckle, the mother's core body temperature rises. Soon enough the mother begins to feel uncomfortable and moves away. Later, desiring another burst of opiates, the rat comes back to the nest, the pups reattach and the cycle begins again.

As an added benefit, maternal hormones may well make the brain more resilient. In 2010 neurobiologist Teresa Morales Guzmán of the National Autonomous University of Mexico showed that the brain of a lactating female is more resistant to the effects of a neurotoxin. The hormones of pregnancy appear to construct a neural shield that protects a mother from damage that otherwise might compromise a rat's ability to care for its young.

Better Connections
The continuous ebb and flow of steroid hormones prompts brain cells to grow many tiny protrusions. Somewhat similar in appearance to thorns on the stem of a rose, these nubs are called dendritic spines. They add surface area to an existing neuron, allowing for more synaptic contact and therefore better information processing. Such spines can grow on a neuron after hormonal stimulation as well as after repeated bouts of stimulation from nearby connecting neurons.

Our lab has built on previous findings from the Rockefeller University showing that dendritic spine densities in the hippocampus increased in concert with the hormonal changes of a female rat's estrus cycle, which is similar to the human menstrual cycle. Best known for its role in memory, the hippocampus also supports maternal behavior. Even after just a few hours of elevated estrogen, the growth was dramatic.

But we learned that the spines are not caused simply by the presence of estrogen. We tested three groups—late-pregnancy females, females treated with a drug that mimics late-pregnancy hormones and females that had recently begun lactating—and saw that all three showed significant increases in dendritic spine concentrations. Unlike the other two groups, lactating females have very low levels of estrogen. We believe that although a mother's hormones initiate spine growth, the process is maintained by the many stimuli a child generates.

With such a thorough remodeling in progress, it is no wonder that many women complain of “pregnancy brain.” The collateral damage of these changes might include an occasionally faulty memory. Human moms experience postpartum memory deficits, too, as work by clinical psychologist J. Galen Buckwalter of the University of Southern California and his colleagues suggests. They found that on cognitive tests of memory for words and numbers, pregnant women and new mothers fared worse than nonpregnant women of about the same age. Their performance on tasks unrelated to child care seemed to suffer.

For the most part, though, the finished product will more than make up for the hiccups a mother may experience as her brain restructures itself. Producing an offspring requires a mother to jeopardize her own health, safety and survival, so her behavioral system kicks in to protect and defend that investment. With the landscape of her brain buffeted by the hormones of pregnancy and pressures of motherhood, she emerges more efficient and geared for survival.

For Liz, the compensation for the downsides of motherhood comes not just from science but also from the heart. By the time we finished writing this article, she had given birth to a healthy baby girl. All the neurobiology in the world pales in comparison to that blissful, ineffable bond that exists between a mother and her baby. Science may explain the maternal brain, but the real marvel—especially when you are gently tucking the blanket around your baby's chin as she sleeps in your arms—might simply be the beauty of a new child's existence.