Having raised eyebrows as a teenager by going vegetarian and swearing off soda, Ms. Rosenfeld took a job after graduating from Brooklyn College at a nutrition center for low-income women, where her devotion to breast-feeding began. “To me, the breast-feeding was not just about better health for the baby, it was about these young girls realizing their baby is important,” she said. “I had 14-year-olds, and here was my opportunity not just to make these babies healthier but to make these mothers caring parents.”

In the decades since, she has taken classes with speech therapists to better understand the mechanics of the mouth. She picks up tips from crazed mothers and calm mothers, women with serious medical problems and women with whom she has to throw up her hands and explain, “Sometimes babies just cry.” She taught a woman with one functioning arm to nurse one-handed and has, with clues that manifest themselves in breast-milk production, detected thyroid problems and retained placenta.

“A hundred years ago, a lot of babies we have today were not born,” she noted. “Forty-five-year-old women having babies, that didn’t happen then; now it does, and sometimes if it’s because of fertility drugs, they’re not going to have milk. I worked with a 1-pound 14-ounce baby who’s a beautiful, delicious child right now. A hundred years ago, that baby would have never survived.”

Typically, new mothers have one session in the weeks after coming home from the hospital, with follow-ups by e-mail or phone. Some call long after they get the hang of nursing, for pediatrician recommendations (“Does it have to be a Russian doctor?” Ms. Rosenfeld asked a Russian-born client the other day), advice on whether to vaccinate, even sleep training. One client recently e-mailed to ask Ms. Rosenfeld’s philosophy on night lights.

She has had celebrity clients, though sometimes Ms. Rosenfeld, who watches only one television show at a time ( “Law & Order” recently made way for “Bones” ) does not recognize them, and other times she pretends not to, because they are, as she put it, “here as a mom.” Once, though, after a client left, she broke into a gleeful chant: “I had an Oscar winner in my house! I had an Oscar winner in my house!” (No, she would not name names.)

The only trouble she had nursing her own children — now 17, 20 and 22 — was giving it up.

“My oldest stopped nursing on his first birthday, and I was crushed,” Ms. Rosenfeld said. “And my husband said, ‘You believe in baby-led weaning!’ ” The youngest hopped into their bed at age 2 and declared, “ ‘I don’t think I need to nurse anymore, Mom.’ I was like, ‘O.K., go ahead, break my heart.’ ”

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Was she drinking? Was she starving? Was that a gulp or just a murmur? Was that an “I’m famished” cry or an “I’m tired” cry? And how do you burp a baby, anyway?