To identify possible sources of cytomegalovirus infection in pregnant women, we studied seven families with a recent case of congenital or maternal cytomegalovirus infection and a history of maternal contact with a young child shedding the virus. We used restriction-endonuclease techniques to compare the DNA of viral isolates collected from family members. Five families contained an infant who had congenital or perinatal infection, a mother who had had evidence of primary infection during her most recent pregnancy, and a child less than three years of age who was excreting cytomegalovirus. All five of the young children attended day-care centers at least part-time. In each of these five families, strains from family members were identical, and it is most likely that the toddler-aged child was the source of the virus for both the mother and the fetus or infant. In two other families, acquisition of cytomegalovirus by children in a day-care center was followed by seroconversion in the mother along with excretion of a strain of the virus identical to that in her child, as demonstrated by restriction-endonuclease analysis. Five of the seven fathers were tested for antibody to cytomegalovirus; four were seronegative, ruling them out as a source of infection in the mothers. These results not only strengthen evidence for the transmission of cytomegalovirus from child to mother but also indicate that infections acquired by a mother from a child can be transmitted to her fetus. (N Engl J Med 1987; 316:1366–70.), RECENT large-scale population-based studies have shown that from 1.6 to 3.7 percent of women acquire cytomegalovirus infection for the first time during pregnancy, and that in 30 to 40 percent of these primary maternal infections, the virus is transmitted to the fetus.12345678 Most babies with congenital cytomegalovirus infection appear to develop normally, but 10 to 20 percent of those infected after a primary maternal infection have sequelae affecting hearing, mental ability, neuromuscular function, or vision.678910 In spite of the obvious importance to public health of cytomegalovirus infections contracted during pregnancy, routes of transmission that lead to these infections have not… © 1987, Massachusetts Medical Society. All rights reserved.