KANSAS CITY, Mo. — Parents who heat plastic baby bottles risk feeding their children a synthetic hormone linked with medical, reproductive and developmental problems, according to a University of Missouri study released Thursday.

The chemical — bisphenol A — is used in making hard, polycarbonate plastic and leaches out of the bottles when heated to 80 degrees or filled with hot liquids, researchers said. Bisphenol A is a synthetic estrogen that can cause feminization in boys, an onset of early puberty in girls, prostate and breast cancer, and some forms of diabetes.

At a news conference, consumer groups said the new Missouri study confirmed earlier warnings and is significant because it examined major brands of baby bottles sold in the United States and Canada.

Approximately 90 percent of all plastic baby bottles sold in the United States are made with bisphenol A.

Mike Schade, a co-author and spokesman for the Center for Health, Environment and Justice, a Virginia-based advocacy group, said "the only appropriate response" to the study is to replace the plastic bottles that contain the chemical with "safer products in order to prevent harm wherever possible."

Industry groups, however, disputed the study's conclusions.

Amy Chezem, a spokeswoman for the Juvenile Product Manufacturers Association, which represents baby-bottle makers, said the findings are not new.

"The industry has been dealing with the issue — it's just scare tactics for parents," she said.

Chezem noted that the U.S. Food and Drug Administration studied the issue and had not found a problem.

"These are tests done on animals and are not equivalent with what happens in real-life scenarios," she said.

The American Chemistry Council, another trade group, issued a statement saying other scientific studies "support the conclusion that bisphenol A is not a risk to human health at the extremely low levels to which consumers might be exposed."

The trade group pointed out that the chemical has been used for years to make bicycle helmets, eyeglass lenses and life-saving medical devices.

Wade Welshons, a University of Missouri professor and researcher who worked on the study, said that "we found surprisingly large levels (of bisphenol A) leaching out" of heat-tested bottles.

"We didn't expect that much I can assure you," he said, noting that older bottles that were washed numerous times posed higher risks. "I agree that this is a large concern."

Welshons said the high levels of chemical leached from the bottles correlated with those shown to cause adverse health effects in test animals. He noted that it is accepted science for animal studies to be used to show potential health effects in humans.

David Carpenter, a public health physician and director for the Institute for Health and Environment at the University of New York at Albany, agreed that there is the potential that children are being exposed "at the most vulnerable time in the early stages of life when the body and brain are developing."

Consumer groups said that parents who are concerned might want to consider switching to glass bottles or discarding older plastic bottles which become cloudy in appearance after multiple washings.

Some states are considering legislation to limit use of polycarbonate plastic. Some bottle makers — such as BornFree — do not use bisphenol A, but their product costs more.