Primary cytomegalovirus infection in pregnancy. Incidence, transmission to fetus, and clinical outcome.

We studied 16 218 pregnant women from two income groups to determine the incidence of primary cytomegalovirus (CMV) infection and its consequences for the offspring. In the high-income group, 64.5% of the women were seronegative for CMV and 1.6% had primary CMV infection. In the low-income group, only 23.4% of the women were seronegative for CMV, but 3.7% experienced a primary infection. The rate of transmission in utero was similar in the two groups (39% and 31%). Congenital infections were more frequent in the low-income group; however, primary CMV accounted for 25% of the congenital infections in this group, in contrast to 63% of the high-income cases. Infections acquired early and late in gestation had similar rates of transmission in utero, but three infants (8%) with symptomatic congenital infection and five infants (13.5%) who have developed significant handicaps were exposed in the first half of pregnancy. Primary CMV infection during pregnancy poses a 30% to 40% risk of intrauterine transmission and adverse outcome is more likely when infection occurs within the first half of gestation.