WASHINGTON — Scientists have begun recruiting mothers-to-be for the largest study of U.S. children — aiming eventually to track 100,000 around the country from conception to age 21.

"We are embarking on the road to discovering the preventable causes of the major chronic diseases that plague American children today," Dr. Philip Landrigan of the Mount Sinai School of Medicine, one of the lead researchers.

Nearly a decade in the planning, the ambitious National Children's Study tackles a major mystery: How the environment — everything from a pregnant woman's diet to a child's exposure to various chemicals — interacts with genetics to affect youngsters' health and development.

Autism, asthma, certain birth defects and other child disorders are on the rise, as is concern about which environmental factors play a role. Plus, many adult diseases take root in childhood.

But while technology has finally advanced enough to separate multichemical and gene-environment interactions, research until now hasn't included enough children to prove why some are more at risk than others. The hope is that the new study can identify both what's harmful and what's not.