A few weeks after the birth of my first son, I caught myself rocking a stick of butter to sleep while in line at the supermarket checkout. I gently moved the shopping cart back and forth, until I became aware of what I was doing and stopped in the hope that no one had observed me.

My son was one of those kids who often got jolted out of his sleep for no apparent reason. His baby carriage stopping at a corner was reason enough. Because of this, I soon learned to push his wagon back and forth when waiting for the light to change or when standing on line. This movement had become second nature to me.

During the first weeks and months after birth, most parents find themselves in a kind of baby mode: their daily schedule is determined by their infant, and their thoughts revolve around his or her needs. Mommy and Daddy do whatever is necessary to keep the baby fed and happy.

But newborns do not develop a day-night rhythm until later, and all of them want to nurse every two to three hours. Postponements are not accepted graciously. Even the most exhausted parents wake up with a start when they hear their baby’s shrill cries.

But for some strange reason, many parents seem not to mind the frequent nighttime wake-up calls. They nurse or feed their baby and change diapers because that is just what you do. Only the next morning, when they see their child sleeping peacefully, do they forget the rigors of the night before. What makes that possible?

Apparently the brain simply makes internal adjustments to these new challenges—an evolutionary adaptation without which mammals would be unequipped to nurture offspring. During the first few years of life, infants are completely helpless and would not survive for long without engaged long-term parental support. No matter how sleep-deprived newly minted parents may be, they prioritize the well-being of their children.

The transformation to a life dedicated to motherhood occurs during pregnancy and the initial postpartum weeks. During this time the mother’s body is flooded with a cocktail of hormones, among them the attachment hormone oxytocin as well as estrogen and prolactin, which stimulates lactation. Some regions of the brain have receptors for these hormones—the areas that deal with stress, for instance. One notable region is the hypothalamus, which controls important functions such as sleep and hunger. A subregion, the medial preoptic area (mPOA), probably plays a critical role in parental nurturing. If this is excised from the brain of mother rats, they suddenly neglect their offspring.

The Neural Engine of the Maternal Instinct

During pregnancy the neurons, or nerve cells, in the mPOA undergo change. Researchers have observed that in pregnant rats these cells grow, and the number and length of the dendrites that conduct signals from other neurons also increase. The researchers achieved the same effect by administering a mixture of pregnancy hormones to virgin animals. In all probability, these hormones prepare this region for birth and motherhood.

At a practical level, parents must struggle to determine what makes a crying baby unhappy. Fatigue? Hunger? Perhaps she wants some attention—or to be left alone. Some mothers have an uncanny feel for what the problem might be. They are able to interpret their baby’s facial expressions correctly and know intuitively how to position themselves so that the infant, who cannot yet focus her eyes, can nonetheless recognize who is there for her.

Oxytocin hormone crystals. Credit: Alfred Pasieka Getty Images

This behavior is the result of numerous adjustments and changes that have taken place within the brain. These modifications affect the regions that enable us to direct our attention and the networks of the reward system. Two important protagonists in this process are oxytocin and dopamine; the latter enables us to take action and increases drive. The mere interaction with the child stimulates the release of both these chemical messengers, thereby intensifying caring behavior.

Studies have shown that women who have high blood levels of oxytocin during pregnancy and the first postpartum months respond empathetically to their children. Duration of gaze, tone of voice, smiles—everything is just right. In contrast, mothers who have low oxytocin levels more frequently exhibit more intrusive and less empathetic behavior.

As experiments have shown, even the most subtle signals, such as a baby’s gurgling or facial expression, have a rewarding and therefore motivating effect on the maternal brain. The power of this feeling of elation was demonstrated by neuroscientist Joan Morrell and her co-workers at Rutgers University. In a thought-provoking experiment, they gave mother rats a choice shortly before giving birth: Would they prefer the presence of three infant animals or a dose of cocaine? Eight days after giving birth most of the mothers preferred the babies. Later, however, that pattern was reversed; after 16 days almost all the animals succumbed to the drug. The rewarding effect of the babies apparently decreases over time—at least in rats.

Nursing stabilizes attachment between a mother and her child because the mother’s body releases large quantities of oxytocin during nursing. Physiologist Craig Ferris of Northeastern University and his colleagues sought to understand exactly what this messenger substance does in the brain. They placed mother rats in a brain scanner while they were nursing their young. They also administered a dose of oxytocin to another group of mother rats.

The brain activity of these rodents was astonishingly similar. The olfactory system and the emotional and reward centers, such as the nucleus accumbens, the insula, the amygdala and various core regions of the hypothalamus, were all stimulated by nursing and by administered oxytocin. If, however the researchers administered a substance that blocks the binding sites for oxytocin, the activity in these areas was considerably weaker.

And in humans? In fact, studies using functional MRI indicate that in women oxytocin sensitizes brain circuits that process emotions and increase motivation and empathy, especially in the limbic system.

It is well established that women release large quantities of oxytocin during vaginal delivery. The hormone causes the contractions in the muscles of the womb that trigger labor. This gave psychiatrist James Swain, now at Stony Brook University, an idea. He and his team placed mothers in a brain scanner and played them audio and videotapes of the crying of their own baby and that of a strange baby. Half of the women had undergone natural delivery a few weeks earlier; the other half had delivered by cesarean section. The result was that the emotion and motivation networks of mothers who had given birth to their children vaginally responded to their own baby’s cries with far greater neural activity than to the cries of a strange baby. Also, their responses were greater than those of the cesarean mothers.

Postcesarean Baby Blues

As researchers have suspected, a lack of oxytocin is also a predisposing factor in postpartum depression after a cesarean section. Mothers who have delivered by this procedure often feel depressed and irritable and demonstrate comparatively less interest in their newborn.

This observation does not, however, mean that a cesarean section permanently damages the mother-child relationship. Swain repeated his crying experiment three to four months later with the same mothers—and the effect that he saw earlier had disappeared. In both groups of mothers, the brain responded equally to the sound of their baby crying. It seems that the initial difference in oxytocin release eventually decreases.

A similar experiment was conducted by Pilyoung Kim, now at the University of Denver, and her team. All his test subjects had had vaginal deliveries, but they differed in how they fed their newborns. One half of the women nursed their babies, whereas the other half bottle-fed them. As expected, the oxytocin released during nursing sensitized the brain’s motivation and emotion centers. The nursing mothers responded more strongly to their child’s cries.

Credit: michellegibson Getty Images

The neural reaction to infant crying changes over time, however. Immediately after the birth of their first child, some mothers begin to sweat in response to the slightest whimper. Did I do something wrong? Is my baby sick? How can I calm him? Of course, such urgent questions are not exactly a rarity among mothers. This initial sense of insecurity is also reflected in brain activity. As Swain noted in his studies, the amygdala and insula respond especially strongly to the baby’s crying during the first two to four weeks postpartum. And these regions of the brain are important centers for the processing of fear.

After three to four months, however, this activity migrates to regions in the hypothalamus and the medial prefrontal cortex. The latter regulates negative emotions by suppressing the amygdala’s reactions to unpleasant stimuli. This change can be seen as mothers become more self-assured and respond more flexibly to their child’s needs.

But neural activity is not all that changes as a result of pregnancy and birth. The “hardware” of the maternal brain adapts to the new challenges. In 2010 psychologist Benedetta Leuner, now at Ohio State University, counted the dendritic spines in a segment of the prefrontal cortex of rats. These mushroomlike protrusions, or spines, cover neurons’ branching structure, the dendrite, where signals are received from other cells. In fact, young mother rats had considerably more spines than virgin animals did, although it is not yet clear how this finding affects behavior. Nevertheless, behavioral experiments seem to indicate that mother animals are cognitively more flexible and less distractible—possibly because their prefrontal cortex is more efficient.

How do researchers define efficiency, though? It is not possible to count directly individual cells or dendritic spines in the living human brain. Researchers must thus fall back on methods such as MRI, where the resolution is not as fine. Elseline Hoekzema of Leiden University in the Netherlands and her colleagues did just that in a study published in 2017.

The researchers determined the volume of gray matter of childless women–which is made up largely of cell bodies of neurons. When 25 of the test subjects became mothers during the following year, their brain was rescanned. To the researchers’ great surprise, the gray matter in some regions of the frontal and temporal lobes actually shrank during pregnancy. This shrinkage affected those regions that enable us to decode the emotions, expectations and intentions of others. These changes were not seen either in the fathers or in the childless control group. Apparently the changes result from the pregnancy itself and not from an adaptation to the parenting role.

According to Hoekzema, this decrease in volume does not necessarily imply a reduction in the number and size of brain cells. What is more probable is that the neural connections undergo restructuring. She compares this process with so-called pruning in puberty, in which excess synapses are removed. This change ensures that neuronal communication grows more efficient among the synapses that remain.

Credit: Sally Anscombe Getty Images

So much for the mothers. Does the paternal brain also adapt to the new role? The answer is a resounding yes. Changes begin during his partner’s pregnancy. As is seen in women, the quantity of blood prolactin and estrogen increases in men, albeit of course at a much lower level. The testosterone level of fathers-to-be, on the other hand, is somewhat lower than in childless men. Testosterone levels remain somewhat lower after the birth, especially in men who spend a great deal of time with their children. At this point, researchers can only speculate as to the reasons. It could well be that lower testosterone levels make fathers calmer and more averse to risky behavior.

As studies on mice have shown, nerve fibers in the brain that respond to oxytocin and vasopressin are more common in mouse fathers. Vasopressin promotes nurturing behavior in female rodents. If researchers block the vasopressin system, the animals exhibit less nurturing behavior toward their offspring. Among other things, these nerve fibers also receive touch signals from the skin. Researchers suspect that a round of cuddling with children may activate a caregiving response.

In 2014 Ruth Feldman of Bar-Ilan University in Israel and her co-workers undertook a study of what exactly takes place in the brain of a father after childbirth. In a brain scanner, the neuroscientists showed test subjects—heterosexual fathers, their wives and fathers from a same-sex relationship—video sequences of their infant. The brain of the mothers exhibited typical excitation of the emotion centers, such as the amygdala and insula. In contrast, brain regions in the cortex of heterosexual fathers that engage in planning and empathizing with others became active.

What was astonishing was that when both parents were men, the “maternal” brain regions of the fathers who had assumed the greater responsibility for child care were also activated, among others the amygdala and insula. With regard to brain activity, they were simultaneously father and mother.

The more time that heterosexual fathers spent alone with their children, that is, without their partner, in the months before measurement, the more their brain recruited the involvement of the amygdala, the typically female component of the neural caregiving network. According to the researchers, the father’s emotional center switches to standby when the mother is a steady presence. In effect, he lets her bear the brunt of the worry. But when he alone is responsible, his brain increasingly takes on this function. And what is the upshot? Dads, let your wives get out of the house more often—chances are you’ll do just fine.