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.