Dyslexia may be caused by abnormal brain development before birth, a Harvard Medical School professor told an audience of about 40 at Winthrop House yesterday.

"The implications are enormous," neurologist Albert Galaburda said. If dyslexia can be diagnosed in small children then proper treatment for overcoming the disorder could begin at an early age, the assistant professor of neurology said.

An estimated 10 to 15 percent of the population are dyslexic. Speculation about the cause of the learning disorder has persisted since it was first diagnosed in 1890.

Galaburda, who heads the Dyslexia Research Laboratory at Harvard-affiliated Beth-Israel Hospital, and another doctor, Thomas Kemper, were the first to provide evidence for the neurological origin of the disorder. In 1979 they first discovered abnormalities in the brains of dyslexics.

Such abnormalities may be caused by damaging antibodies which enter the brain of a fetus, Galaburda said yesterday afternoon.

"Similar antibodies can damage other natal tissues," the neurologist said, adding that children of mothers with lupus have an increased chance of developing congenital heart block. The conclusion that antibodies can also damage brain tissue is "a very small step," Galaburda said.

Abnormal brain structure in dyslexics may also be caused by excessive levels of the hormone testosterone in the fetus.