After long days discussing America's obesity problem, Melinda Sothern has had enough of windowless conference rooms.

After long days discussing America's obesity problem, Melinda Sothern has had enough of windowless conference rooms.

"I need to exercise," she says, pausing to review her plans in the San Diego Convention Center lobby. She plans to rent a bicycle in Coronado, Calif., and ride, fast and far.

Sothern, 55, is a woman who practices what she preaches. And one of her messages about obesity is aimed at women like herself: mothers.

Fat mothers. Thin mothers. And especially mothers-to-be.

A leading fitness and nutrition expert at Louisiana State University, she has a theory that the tide of obesity that has swept the nation in the last two decades had its roots in what young mothers did, or didn't do, in the postwar, suburban-sprouting 1950s.

If she's right — and evidence is stacking up on her side — reproductive-age women may become the central focus of efforts to reverse America's fat problem.

The obesity epidemic has multiple causes, Sothern acknowledges. Food has changed in the last five decades. Americans have become much more sedentary. But she thinks that obesity rates soared just when they did — in the 1980s — because a generation of young women decades earlier smoked, spurned breast-feeding and restricted their weight during numerous, closely spaced pregnancies.

"It was the evil '50s. A perfect recipe for obesity," she says.

Sothern calls her theory "the obesity trinity." And she thinks the key to getting Americans to slim down lies in studying those lessons from the past. Among her prescriptions for change: Women who are significantly overweight should be discouraged from having babies until they shed some pounds.

A central part of Sothern's theory — that obesity starts in the womb — is gaining currency with a growing number of doctors and researchers who say that reversing the epidemic, with its attendant cases of weight-related illnesses such as diabetes, should begin by addressing nutrition in pregnancy and early-life feeding practices.

"We don't completely understand how people become obese, when people become obese and why children become obese," says Michael L. Power, a senior research associate at the American College of Obstetricians and Gynecologists and a coauthor of the book "The Evolution of Obesity."

"But children of parents in the '50s and '60s may have started this off."

Women in the 1950s and 1960s — think Betty Draper on the hit TV show "Mad Men" — were generally advised to restrict weight gain in pregnancy to as little as 10 pounds. Inadequate nutrition in some of these women could easily have programmed their babies to catch up on growth during infancy — and studies suggest such growth spurts increase the risk of later obesity.

Women smoked with abandon, unaware of the health risks. Smoking during pregnancy is thought to contribute to obesity risk in offspring because nicotine disrupts mechanisms in the body that control appetite, metabolic rate and fat storage.

Lasty, by the mid-1970s, breast-feeding in the U.S. had hit an all-time low of 25 percent. Studies show that formula-fed babies have a higher risk for obesity than breast-fed babies, perhaps because of metabolic changes or because drinking formula from a bottle is passive and easy and generally done till a bottle is empty.

And since breast-feeding can prevent ovulation, women using formulas were more apt to experience multiple pregnancies over a shorter period of time.