If there were an equation to figure out who gets fat and who doesn’t, most of us would structure it like this: a plate of brownies plus a bag of corn chips times sitting on the couch divided by genetics equals the chance you’ll have extra bulges around your midsection.

But scientists say there could be another step to the equation, a step that makes it more likely someone will gain weight as an adult. And it may even happen before we’re born. The culprit? Plastic, as in plastic baggies, water bottles, containers and even toys.

Researchers say some of the chemicals in plastic could act as endocrine disruptors when small amounts are absorbed by the body. The chemicals mimic hormones and fiddle with your body’s fat cell production schedule. Studies in animals have shown that prenatal exposure to low levels of these chemicals leads to more fat cells, and consequently, more fat.

Centers for Disease Control and Prevention researchers found one of the chemicals, called called bisphenol A [bye-sfeen-ahl A] in the urine of ninety-three percent of the two-thousand people they studied as part of a nutritional survey.

Investigators at the University of Misssouri-Columbia have found that mice exposed to the chemical during development wind up packing on more pounds than other mice.

But don’t panic over plastic yet. Researchers say more studies are needed to evaluate the actual effects of these chemicals.

For now, just keep the corn chips and brownies out of the equation.