City officials cited evidence in The American Journal of Maternal/Child Nursing and elsewhere that more than 80 percent of 10-year-old girls are afraid of being fat, that girls’ self-esteem drops at age 12 and does not improve until 20, and that that is tied to negative body image.

The campaign was conceived by an aide to Mr. Bloomberg, Samantha Levine, 38, the mayor’s deputy press secretary, who is serving as project director. Ms. Levine said she had been moved by stories of little girls wearing body-shaping undergarments and getting plastic surgery to improve their appearance. She said she had also been galvanized by reading the advice columnist Cheryl Strayed, who said a failure of feminism was that women still worried about what their buttocks looked like in jeans.

“I think being a woman in this society, it’s sort of impossible to not be aware of the pressures there are around appearance, around weight, around trying to always look a certain way,” Ms. Levine said.

The idea so resonated among her colleagues that all 21 girls pictured in the campaign are the daughters of city workers, friends and friends of friends, who believed it was important to participate. None are professional models. All but one, who lives on Long Island, live in New York City, she said.

DeVoray, the girl in one of the ads, who aspires to be either police commissioner or the first black female president, said in an interview on Monday that some of her friends asked her if they were pretty. “I say you’re beautiful even if somebody tells you you’re not,” she said. “You have to keep your head up, don’t let anybody bring you down.”

Her mother, Twanna Cameron, a project coordinator for NYC Service, the agency that promotes volunteering, said she had eagerly stepped forward for the campaign. “I think every mom has those worries,” she said. “We can’t all be models, we can’t all be superthin.”