Congenital cytomegalovirus (CMV) infection rates increase with maternal seroprevalence due to transmission from maternal non-primary infection. CMV seroprevalence estimates of pregnant women are needed for planning strategies against congenital CMV transmission. We aimed to determine the age-specific prevalence of serum antibodies for CMV in a representative age-stratified sample of unselected pregnant women from a Brazilian population. A total of 985 pregnant women, aged 12–46 years (median 24 years), were enrolled. Overall CMV seroprevalence was 97% (95% confidence interval 95·8–98·0), with age-specific (years) prevalence as follows: 12–19 (96·3%), 20–24 (97·7%), 25–29 (97·1%), and 30–46 (96·7%). CMV seroprevalence is almost universal (97%) and is found at similar levels in pregnant women of ages ranging from 12 to 46 years. Because high CMV seroprevalence is found even in women of a younger age in this population, this finding suggests that the majority of primary CMV infections occur early, in infancy or childhood. As a consequence, vaccines currently under development to prevent primary infection may not be a solution for the prevention of congenital CMV infection in this population.