In a bid to unravel the confounding mysteries of autism, researchers yesterday launched a comprehensive study to track families with an autistic child through another pregnancy, birth, and the following three years.

Hundreds of environmental factors -- from diet to infection, pesticides and medications -- will be examined for possible interplay with genetic makeup. Perhaps 100 children with autistic spectrum disorder will be born, researchers said, to the 1,200 women they hope to enroll at four study sites nationwide, including Philadelphia.

"These families know there is susceptibility, because they already have an affected child," said principal investigator Craig J. Newschaffer, a professor at the Drexel University School of Public Health, during a teleconference with reporters yesterday.

About one out of every 150 children is diagnosed with autism spectrum disorder, a group of neurological disorders that causes delays in language development and impairment in the ability to relate to others.

For siblings of children with autism, the risk may be 10 times greater -- one reason that genetics is believed to play a large role.

While diagnoses have increased dramatically in recent decades, it is unclear how much of that is due to increased public awareness and better testing.

Some parents believe that childhood vaccinations, which also have increased in recent years, could be a trigger. Medical research has found no solid basis for this idea, but vaccines are among the many environmental factors that will be examined in the new study.

The effort is unusual because "we are collecting the information in real time," said Lisa A. Croen, an epidemiologist with the Kaiser Permanente Division of Research in Northern California, which is part of the study. So-called prospective research is considered more accurate than medical studies that enroll people after a diagnosis and ask them to look back.

Daureena Williams, 26, of West Philadelphia, has two sons -- a 2-year-old with autism and a 1-year-old without -- and has told investigators that she wants to be part of the study if she gets pregnant again. "I just want to know where autism comes from," she said last night.

Anthony Shawn Williams Jr. was born in August 2006. While something seemed wrong within weeks, at 10 months "he started talking, saying 'Mommy' and 'Daddy,'" she said. Then he had the first of two seizures, and he was later diagnosed with autism.

Although he has learned to recite the letters of his name, Ms. Williams said, he waves his fingers and arms in a pattern associated with autism, and his communication is no longer spontaneous. "He would never say, 'Mama, juice!'" she said.

The Early Autism Risk Longitudinal Investigation (EARLI) will include "multiple encounters with mom" during pregnancy, Dr. Newschaffer said, to get blood and urine samples (for DNA and other analyses) and updates on detailed diaries the women will be asked to keep about their health and environmental exposures.

Other data will be collected from the father and from siblings who have autism spectrum disorder. Stool and other samples will be taken from the baby at delivery, followed by examinations at 6, 12, 18, 24, and 36 months.

Researchers will follow a broad range of developmental and cognitive measures to pinpoint the timing of any symptoms that might hint at developmental delays, said Rebecca Landa, director of the Kennedy Krieger Institute's Center for Autism and Related Disorders in Baltimore, which is participating in the study there along with Johns Hopkins University.

Besides Philadelphia and Baltimore, there are two study sites in Northern California, one of them at the University of California, Davis.

The study is funded with $14 million from the National Institutes of Health and $2.5 million from Autism Speaks, an advocacy group.