Fetal alcohol syndrome is a lifelong disorder with physical, mental and behavioral problems spilling over from childhood into adulthood, the first systematic follow-up of such patients showed Tuesday.

Fetal alcohol syndrome, or FAS, occurs in babies whose mothers abuse alcohol when they are pregnant. The syndrome, whose physical symptoms include facial malformations and small head size, is considered the leading known cause of mental retardation in the United States.Although FAS was first identified in the early 1970s, "what the physical and mental manifestations of the syndrome in adolescents and adults are has remained an important unanswered question," a U.S.-Canadian research team said.

In their study, billed as "the first systematic follow-up" of FAS patients beyond childhood, the researchers tracked 61 FAS patients into adolescence or adulthood.

The researchers said their findings confirmed the suspicion that FAS is not just a childhood disorder, showing "there is a predictable long-term progression of the disorder into adulthood."

FAS is currently considered the leading known cause of retardation in the United States, surpassing even Down's syndrome and spina bifida, researchers said.

At least 5,000 U.S. babies are born each year with FAS, said Dr. Reed Tuckson of the March of Dimes. "Fetal alcohol syndrome is completely preventable. We urge women in the strongest possible terms not to drink alcohol while they are pregnant."