Philadelphia ? The federal government on Thursday launched its most ambitious study ever of the health of America’s children, with the plan to follow more than 100,000 from womb to adulthood.

The National Children’s Study, which will start in six communities nationwide, will explore the many environmental influences that bear on children’s well-being.

The study, which will begin enrolling participants in 2007, will look not only at traditional environmental factors such as air and water pollution, but also at issues such as neighborhood safety, family relationships and diet. It will examine the connection between environmental and genetic factors.

The study, projected to cost $2.7 billion over about 25 years if fully implemented, also will attempt to explain racial and ethnic disparities; for instance, why African-American babies are more likely than white babies to be born prematurely and underweight, and why American Indian children are at high risk for diabetes and injuries.

The ultimate goal of the research is to prevent diseases and come up with better ways to treat them, federal health officials said.

“In their search for environmental influences in human health, study researchers plan to examine such factors as the food children eat, the air they breathe, their schools and neighborhoods, how often they see a health care provider and even the composition of the house dust in their homes,” Duane Alexander, director of the National Institute of Child Health and Human Development, said at a Washington news conference Thursday.

The study comes amid growing concern that today’s children are growing fat and coming down with diseases such as diabetes and high blood pressure that used to be considered adult worries.

Both pregnant women and women who are not pregnant will be asked to be in the study, so that researchers can examine factors that may play a role in lifelong health even before conception takes place. In some cases, the women’s partners will also be asked to participate.

“The good news is that 82 percent of our nation’s 70 million children are in very good or excellent health,” U.S. Surgeon General Richard H. Carmona said at the news conference.

But he said there are troubling trends taking hold, including the fact that 15 percent of American children, about 9 million of them, are overweight or obese.

Researchers will analyze blood, urine and other samples for exposure to chemicals; administer questionnaires; and do exams to track children’s growth and development up to age 21.

“The study will help us map how our environments, habits and activities affect our children’s health,” Carmona said.

The study was mandated by Congress in 2000. But money to implement it has been very limited. Over the past several years, the federal agencies leading the study have pooled money to do the planning, about $12 million in all this fiscal year.