Although Balzano didn’t know it when she made the appointment, the clinic that would have performed the procedure had a policy against providing I.V.F. to patients with a body mass index above 45; Gina’s was 51.2. In that decision, it followed much of the fertility industry, including half of the 20 largest clinics in the United States, according to FertilityIQ, an online clearinghouse of information on fertility providers nationwide. At some clinics, the cutoff for treatment is a B.M.I. of 50, often classified as “extreme” or “severe” obesity (roughly 300 pounds for a 5-foot-5 woman). At others, it’s much lower. Chelsea Ritchie, now the mother of twins in Ham Lake, Minn., got a call from a nurse the day before her initial appointment with a fertility doctor in 2011. “She said, ‘The doctor only sees patients with a B.M.I. under 30, so you’ll need to lose 22 pounds,’ ” Ritchie recalls. (The doctor told me that his cutoff for seeing patients is actually a B.M.I. of 35, though he won’t do I.V.F. unless they’re under 30. Ritchie subsequently conceived her twins after going to a different clinic.) B.M.I. doesn’t factor in gender, age or muscle mass, all of which influence body composition and health. But the World Health Organization adopted the B.M.I. scale as an official classification in 1995, and it has since become medicine’s standard metric for categorizing patients by weight. A B.M.I. of 30 or above is classified as “obese,” the word still used by doctors, researchers and the media, although surveys of larger patients show that most consider it derogatory; many now reclaim the once-offensive “fat.”

The belief that a high body weight causes infertility, and its corollary — that weight loss is necessary to resolve infertility — underpin almost every interaction a heavy woman will have with the reproductive health care industry. Yet the specialty’s two governing organizations, the American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology, have not established any guidelines on whether treatment should be declined on the basis of weight. And as the percentage of American women categorized as obese has grown to 41.1 percent in 2016 from 25.4 in 1994, some doctors are pushing back against the notion that weight loss should ever be, in effect, a prerequisite for motherhood. “I think we’ve been overexaggerating the benefits of preconception weight loss,” says Dr. Richard S. Legro, a professor of public health services and chair of obstetrics and gynecology at Penn State University. In fact, a fixation on weight may be leading health care astray. “Many providers see a larger woman and say things like, ‘Don’t eat cheeseburgers,’ even though she’s a vegetarian,” says Sharon Bernecki DeJoy, the director of public health at West Chester University who studies maternity care in the United States. “There’s a lack of recognition of evidence that shows you can be healthy and still have a, quote-unquote, unhealthy B.M.I.” And a lack of recognition that when a heavy person does get sick, it might not be because of weight.