14 Fun Facts About the Science of Motherhood

Mothers are so familiar that sometimes their mysteries are overlooked. As I dove into the research for my new book Mom Genes: Inside the New Science of Our Ancient Maternal Instinct, I began to see that mothers are even more extraordinary than I’d thought. Blue whale mamas produce 50 gallons of milk per day. Human pregnancy may actually be contagious. A woman’s brain is a key organ of childbirth. Many of the most dramatic maternal transformations happen internally and scientists are just now starting to figure them out. I’ve combed through the latest science to share some of these amazing changes with you.

Here are 14 fascinating facts about moms that have been burning a hole in my pocket, which, because I have four kids of my own, is also filled with last year’s crusty Kleenex wads and Cheerio crumbs.

Many Mammal Mamas Carry Kids on the Left

Regardless of whether they are left- or right-handed, human moms tend to cradle their babies on the left side of their bodies, especially in the early months. This left-handed bias likely has to do with the human brain’s lopsided layout: sensory information on the left side of the body is processed on the right side of the brain. The brain’s right hemisphere is also where emotions are processed, so holding and observing the baby on the left may help transmit social information to the right side more efficiently. Babies seem to prefer to keep their mother in the left visual field, too. Fascinatingly, researchers recently documented left-side bias in non-primate mammal mothers. Observed off the coast of a Russian island, walrus moms tend to keep their babies on the left while bobbing along the waves, and their calves swam over to their mother’s left side before diving to suckle. Ditto for flying fox moms dangling from tree branches in Sri Lanka who seemed to favor keeping their babies on the left.

“Mommy Brain” Is Real—and Very Complex

Roughly 50 to 80 percent of moms report what’s sometimes called “mommy brain,” the brain fog and mental bloopers associated with pregnancy and new motherhood. Individual experiments offer conflicting evidence, but a 2018 meta-analysis of 20 studies found that memory problems and poorer executive functioning do seem to be common themes, starting in the first trimester and worsening through the third.

In a first-of-its-kind, ground-breaking analysis of pre- and post-pregnancy brain scans published in 2016, researchers found mothers lose gray matter during pregnancy—and these losses endure for at least two years. But volume loss may come with some benefits, too. The brain zones used for processing and responding to social cues might get more efficient in pregnancy, as the women who suffered the biggest gray matter losses scored higher on a standard assessment of a mother’s attachment to her child.

Women Pregnant With Boys May Get Nauseous More Easily

Snips, snails, puppy dog tails…yuck. Pregnant women carrying boys are measurably more sensitive to disgust, at least one rather creative study found in 2015.

A pair of Polish researchers studied disgust sensitivity in 92 pregnant women during all three trimesters using the “Disgust Scale” questionnaire, a commonly used assessment in psychology studies evaluating the emotion. The test is loaded with ick-inducing descriptions to vet a respondent’s reaction to cockroaches, watching someone eat “ketchup on vanilla ice cream,” hearing someone clear a “throat full of mucous” and seeing “a human hand preserved in a jar.” Mothers carrying sons had higher disgust sensitivity compared to mothers carrying daughters in the first trimester. While girl-moms’ queasiness decreased during the second trimester, boy-moms actually experienced elevated stomach-turning reactions.

Don’t Mess With Animal Moms—Even Squirrels

YouTube videos of beastly moms abound—from a mother moose charging grizzly bears to a mountain lion mama swatting at a terrified jogger who stumbled upon her cubs. Scientists have also studied maternal aggression in slightly less formidable animals: ground squirrels, who ferociously defend their youngsters by kicking gravel at rattlesnakes. Researchers played the sounds of fake rattlesnakes and found that squirrel moms—compared to non-mothers and males—were especially reactive to the ominous rattling. Squirrel moms with the youngest babies took extra risks to protect their newborns in a second experiment.

The widespread phenomenon of maternal aggression may involve oxytocin, a neurochemical also related to birth and lactation. In a 2017 lab experiment, rat moms stopped attacking a threat once oxytocin signaling in part of their brains was blocked.

Mother Cows Are Especially Defensive

Cows were recently declared the most dangerous large animals in Britain, killing more people than dogs—74 over a span of 15 years. Some of these rampaging bovines were bulls, but many were mother cows. Most victims were farm workers, but passersby also ran afoul of the cow moms, which is why the government is begging farmers not to pen naturally aggressive new mother cows in publically accessible fields, where hapless human walkers may be mistaken for calf-hungry predators. Dog walkers especially may provoke the attacks—in 17 out of 18 human walker deaths by cow, dogs were involved. Even non-fatal cow attacks amount to a type of “high-velocity trauma,” a ten-year review of hospital injuries found.

Girl Calves Have It Good

Some mammals produce richer milk for their sons, perhaps because large male body size is ultimately more important in mate competition. But a study of nearly 1.5 million Holstein cow moms showed they churn out more milk for daughters, to the tune of hundreds of extra gallons per year per cow. Scientists aren’t sure why, but the extra rations might help the female offspring reach sexual maturity earlier and thus have longer reproductive careers. This milky signaling seems to happen prenatally, since calves are often taken away from mother cows a day after birth in the dairy industry, but their mothers still produce extra-ample milk.

Sea Otter Moms Nurse Themselves to Exhaustion

Lactation is a major drain on mammalian moms. Sea otter moms have exceptionally high energy demands, because of their small body size in the heat-sucking Pacific. They are notoriously vulnerable to massive depletion of energy reserves in the months after pregnancy, when they are simultaneously feeding their pups and themselves, foraging half the day in a quest to eat a quarter of their body weight. The result is a state of “utter exhaustion” that scientists call “end lactation syndrome”—which likely explains why so many postpartum otter moms mysteriously succumb to minor infections and incidental wounds. When scientists studied a lactating captive otter named Clara, they found that in the period after birth when she was nursing her pup, her energy demands more than doubled: if that happened in the wild, she would likely become more vulnerable to danger, from disease to resource scarcity.

Blue Whale Moms Produce 50 Gallons of Milk Per Day

As the largest living mammals on Earth, blue whale moms have a big job to do. Once their calves are born, the fast-growing giant babies gain 200 pounds per day. To provide enough sustenance to reach their adult weight of up to 400,000 pounds, blue whale mothers produce 50 gallons of milk per day with between 35 and 50 percent fat content. Researchers are using tiny samples of blubber to learn more about how these humungous mothers pull off such an incredible feat. Hormonal fluctuations in mother whales’ enormous fat stores may be a valuable research tool, according to scientists who take blubber biopsies to learn about mysterious and critically endangered species like North Atlantic right whales. They’ve developed a “library” of these lard samples, each of which is about the size of a pencil eraser.

Moms Have Been Using Bottles for a Really Long Time

Human moms have likely been bottle-feeding since prehistory. Analyzing ancient clay vessels from child graves in Germany, scientists recently found the residue of milk from hoofed animals and identified the vessels as primitive baby bottles, the earliest dating back more than 7,000 years. The Bronze and Iron Age bottles that the scientists sampled looked more like round spouted bowls—or some might say, breasts. A few also feature animal feet and other decorations, suggesting that they might have doubled as baby toys. Scientists have speculated that the advent of bottle-feeding may have allowed local mothers to resume ovulation, which is frequently halted during nursing. This might in turn help explain certain previously mysterious Neolithic baby booms.

The Ice Age Made Mothers Evolve Better Breast Milk

Scientists suspect that a tweak to human moms’ breast tissue helped some populations survive the last ice age. Roughly 20,000 years ago, vital vitamin D would have been increasingly difficult for babies dwelling at far-northern latitudes to harvest through sunlight and exposed skin. Luckily a genetic mutation arose in mothers’ breast ducts that some scientists think allowed for critical nutrients to flow into infants in vitamin D-deficient conditions.

Bug Moms Serve Snacks, Too

Mammals are perhaps the most involved animal moms, yet a small but distinguished number of creepy crawlies are also doting mothers. Mommy daddy long legs tote their spiderlings for a week after they are born. And one type of earwig mom gives her all, Her hatched offspring completely consume their mother—a chilling process called matriphagy.

Dolphins May Teach Babies Sounds Before They Are Born

Bottlenose dolphin moms-to-be start whistling more often about two weeks before they give birth, according to scientists who tailed a mom-baby duo at Six Flags Discovery Kingdom in Vallejo, California. Because dolphins have unique calls, this uptick in vocalizations may have been an effort to teach the baby dolphin her mom’s signature whistle while she was still in the womb. Interestingly, research into human mothers’ vocalizations suggests that we have signature tunes as well.

Moms May Be More Vulnerable to Tooth Decay

The old wives were on to something when they warned “gain a child and lose a tooth.” Women who have had three children forfeit four chompers more than those who have had two kids or fewer. Women whose first two children are the same sex, and who then go on to have a third child, are particularly at risk. Problems with gum disease and calcium absorption in pregnancy may leave moms vulnerable—and so might all those missed dental appointments, which might be a particular problem for mothers juggling multiple young children.

Pregnancy Might Actually Be Contagious

Analysis of the pregnancy timing of more than 30,000 German women found that pregnancy spreads in workplaces: In the year after a colleague had a baby, there was an uptick in first pregnancies in the same office. And families are contagious too. A Norwegian study of more than 110,000 sibling pairs shows that siblings have a relatively strong influence on each other when it comes to first pregnancies.

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