What’s a ‘Normal’ Amount of Time to Breastfeed?

It’s a question that often plagues new parents, but clues from our evolutionary past can help explain why we wean and when.

Credit...Alex Citrin

This story was originally published on Sept. 3, 2019 in NYT Parenting.

I never intended to be one of those people who would whip out a boob for a toddler old enough to demand milk using full sentences. When I was pregnant, I hoped I’d be able to breastfeed at all. If things worked out, I thought, I’d continue for a normal amount of time and stop before it got, you know — weird.

But what’s a normal amount of time to breastfeed? The American Academy of Pediatrics and the World Health Organization suggest feeding a baby only breast milk, if you can swing it, for six months. After that, the A.A.P. recommends supplementing breast milk with solid foods until age 1; the W.H.O. goes further by recommending some breastfeeding along with solids until age 2 or beyond.

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But there’s nothing like delivering a child to remind yourself that whichever guidelines you read on your computer screen, you’re still an animal driven by biology. Since mammals first evolved some 200 million years ago, they’ve come up with countless ways to nurse their young: underwater, upside-down, a dozen at a time. The echidna — an egg-laying mammal that looks like a hedgehog with a long snout — has no nipples at all. She leaks milk straight out of skin patches in her abdominal pouch for her baby to lap up.

What is “normal”?

Turns out, the average human mother is a pretty weird kind of mammal. We’re driven to make decisions not just by our biology, but by the judgments of ourselves and others, and the flexibility of our social structures. And compared with our closest relatives like chimps and orangutans, what makes humans unique — and possibly so successful as a species — is not so much how we nurse, but how we stop nursing.

My daughter and I had no trouble breastfeeding. As an infant, she hated many things, loudly, but loved nursing. That enthusiasm didn’t fade as we passed her first birthday. I nursed her at singalongs, on a squash court at a college graduation, and on airplanes, with my nipple inches from a stranger’s elbow. We kept this up for well over two years. Many of my friends, meanwhile, started to wean their own babies and toddlers when they were around 1.

[For more on breastfeeding, see our guides on how to breastfeed during the first two weeks of life and how to wean.]

Daniel Sellen, Ph.D., an anthropologist at the University of Toronto, was quick to remind me that while breastfeeding my child into her toddler years may have seemed unusual among my peer group, my global perspective on nursing — as a white, college-educated, heterosexual American mom — was limited. In many parts of the world, for instance, it’s perfectly normal for mothers to openly nurse a child who’s 2 or 3. And while it’s true that fewer than 16 percent of moms in the United States are still nursing at 18 months, longer nursing is more common in certain pockets of the country — “Your Davises, your Portlands, your Austins and so forth,” said Dr. Sellen. These areas tend to house more highly educated people and have stronger social support systems for breastfeeding.

In a study published in The Journal of Nutrition in 2001, Dr. Sellen reviewed data on weaning patterns from 97 modern and historical non-industrialized cultures and found that while specifics varied widely — early-20th-century Tibetans weaned between 10 and 12 months, for instance, while the Native American Arapaho of the late 1930s nursed for four years or longer — mothers, on average, stopped nursing when their babies were around 2.5.

But to better understand why humans wean at the times we do, it’s helpful to look to ancient humans and our nonhuman primate relatives, which can give us clues, “potentially, into the whole arc of our development,” said Tanya Smith, Ph.D., a biological anthropologist at Griffith University in Australia.

In the past decade or so, researchers have learned more about other species’ weaning ages by analyzing the chemical signature of nursing in their teeth. Growing humans and other animals lay down dental tissues in fine layers each day, like tree rings, starting before birth. These layers hold a precise record of our milk (or formula) consumption.

In one study published in 2017, Dr. Smith and her colleagues analyzed the teeth of wild orangutans, whose treetop nursing is hard to observe, and found that the apes fed their young milk for eight years or more. Chimpanzees — our closest living relatives — don’t finish weaning until their young are around 4 or 5.

But while apes seem to nurse for longer than human babies, Neanderthals (our extinct cousins) may have weaned more like modern humans. In a 2018 study of a Neanderthal tooth, for instance, Dr. Smith and her colleagues found evidence of gradual weaning at age 2.5. In other words, weaning when we do may have been an important step in human evolution.

We had dabbled in baby sign language starting around 9 months, and my daughter’s first — and favorite — sign was “milk.” Embarrassingly, the sign for milk is to squeeze your hand as if milking a cow. At 18 months, she liked to hold a stuffed animal up to my breast, make a little smacking sound, and then say “Thank you!” By 22 months, she had learned to say “Mommy’s.” As in:

Her: (Squeezy hand.)

Me: “You want some milk in a cup?”

Her: “No, Mommy’s.” (Diving inside my shirt.)

Were things getting weird? A little.

Meredith Brockway, Ph.D., R.N., a nurse and postdoctoral researcher studying clinical uses of human milk at the University of Manitoba, said that my story is common. In her experience, most moms who find themselves breastfeeding into toddlerhood didn’t plan it that way. And it’s normal to feel judged for it.

After the W.H.O. published its recommendation to nurse until 2 or beyond in 2003, for instance, Dr. Brockway noticed that several of her fellow public health nurses “were really put off” by the suggestion to breastfeed for that long. “Some of them said it was gross.”

Bothered by such negative attitudes, Dr. Brockway set out to learn more about the stigma that is often attached to longer breastfeeding. In 2016, she and her co-author published a review of studies about the experiences of moms who breastfed past 1 year, finding that they “included stigma and secrecy.” Some mothers even hid their nursing from their partners.

Dr. Brockway said she believes it should be up to each mother and baby to negotiate a time to wean. Biologically speaking, there isn’t one right weaning age for everyone, said Katie Hinde, Ph.D., an evolutionary anthropologist at Arizona State University who studies lactation. In a 2015 study, Dr. Hinde and her co-authors looked for consequences of weaning at different ages. They compared 231 children in Tanzania who had nursed for two or more years with 84 children who’d stopped nursing sooner. In terms of growth and immune function, the two groups were essentially the same.

While this study didn’t find any specific benefits to breastfeeding past age 2, it also didn’t conclude that it’s pointless. Perhaps there’s a hidden biological cue passing between baby and mother that encourages weaning after children have reached some developmental threshold, whatever age they are. The data only show that the choices of each mother and child in that study led them to pretty much the same outcome.

I didn’t want to wean my daughter before she was ready. But after her second birthday, I stopped offering my milk in the morning, to nudge things along. At 28 months, she went several days in a row without nursing at all, then asked for milk during a cranky bedtime. I let her try, but she frowned while she sucked; nothing seemed to be coming out. I told her the milk might be all gone.

She sat up. “I just drank it all!” she said, looking a little stricken. Then she went to bed, and that was that.

We were able to wean at a time that worked for both of us, but not all moms are so lucky. The structures of our societies make it more or less — often, less — feasible to breastfeed. Women have to work, or don’t have support from people around them. Others experience stigma for nursing toddlers or nursing in public, while still others are judged for not nursing at all.

But flexibility in how we feed our babies, as Dr. Sellen’s study across cultures showed, is part of being human. And compared with our mammalian relatives, our ability to wean at relatively earlier ages might have been key to the evolution — and success — of the human race.

Unlike orangutan, chimpanzee or gorilla moms, who have to do everything themselves, according to Dr. Hinde, ancient human moms got help from other people — fathers, grandmas, siblings. As we evolved to wean sooner and share child care duties, infants became less costly to their mothers. That may have meant we could have more babies. And evolutionarily speaking, more young means a more successful species.

“Humans can have more babies, closer together, that survive at twice the survival rate of other wild apes,” Dr. Sellen said. Hundreds of thousands of years ago, that difference may have given our species a leg up. Today humans are the planet’s dominant ape, no matter how we feed our children.

Breastfeeding may make us mammals, but weaning is part of what makes us human.

Elizabeth Preston is a science journalist in the Boston area.