Washington ? Does a pregnant woman’s exposure to certain chemicals put her child at risk of learning disabilities? Do genetics and pollution interact to cause asthma? What’s the real impact of TV on toddlers?

The government is preparing the largest study of U.S. children ever performed — it will track 100,000 from mothers’ wombs to age 21 — to increase understanding of how the environment affects children’s health.

It’s called the National Children’s Study, and pediatric specialists say it is coming at a crucial time. Rates of autism, asthma, certain birth defects and other disorders are on the rise, as is concern about which environmental factors play a role. And technology has advanced enough to allow study of multichemical and gene-environment interactions that might explain why some children seem at greater risk.

The study “really represents our generation’s best hope of coming to learn the environmental causes of these conditions,” says Dr. Philip Landrigan, an expert on children and the environment at New York’s Mount Sinai School of Medicine.

It’s a quest to prove both what’s harmful, and what’s not.

“There are things we probably should be worrying about that we’re not worrying about enough, and there are things we’re worrying about too much,” adds Landrigan, who is advising the National Institutes of Health on the study’s design.

The study, ordered by Congress in 2000, is in its late planning stages. Enrollment of pregnant women is set for 2006, although proponents hope three pilot sites could begin work late next year. Already, families alerted by interested patient-advocacy groups are asking how to participate.

The question is money. Scientists say they need $27 million to $50 million next year to ramp up, including hiring a laboratory big enough to store more than 2 billion anticipated biological and environmental samples — from participants’ blood and DNA to dust from their houses, soil from their yards and air from their neighborhoods.

Congress has provided roughly $12 million annually for three years of study preparation. President Bush requested the same amount for next year, and budget constraints have lawmakers indicating they’re unlikely to provide more.