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.