NEW YORK, NOV. 7 -- Doctors say they have uncovered seven infants with syphilis after initial tests of the baby, the mother or both showed no traces of the potentially fatal venereal disease at birth.

The initial tests probably failed to detect syphilis because at the time of birth the infection was recent, said David Dorfman of the Bronx Municipal Hospital and Joy Glaser of the Albert Einstein College of Medicine.

The researchers, writing in Thursday's New England Journal of Medicine, said doctors should be on the lookout for syphilis even if tests done just after birth are negative.

Dorfman and Glaser uncovered the seven cases of missed syphilis in a search of hospital records from December 1988 through November 1989. The cases were discovered when the babies returned to the hospital to be treated for a variety of symptoms, including fever or a rash on the palms and soles. All were between three and 14 weeks old.

The study comes at a time when syphilis cases are growing dramatically, apparently fed by the problem of crack cocaine.

The fact that pregnant addicts may use prostitution to feed their habits may explain why the number of babies with syphilis in New York jumped from 57 in 1986 to more than 1,000 in 1989.

"With increased attention being paid to the detection and treatment of sexually transmitted diseases in New York, it was surprising to us that seven infants beyond the newborn period were admitted during the past year with symptomatic but previously undiagnosed congenital syphilis," the researchers said.

Although some doctors may believe a single test of the mother during pregnancy is sufficient, they said, mothers and babies also should be tested at the time of delivery.

Doctors also should suspect syphilis if an infant is suffering from a fever with aseptic meningitis (an infection of the membrane surrounding the brain and spinal column), an enlarged liver or blood abnormalities, they said.