Congenital CMV infection is caused by in utero mother-to-fetus transmission and is a leading cause of birth defects and developmental disabilities. The highest risk of disability is to children born to women who have a primary infection during pregnancy, which can be detected by measuring seroconversion. We reviewed studies that reported rates of CMV seroconversion in different populations. Among pregnant women, annual seroconversion rates typically ranged from 1 to 7% (summary annual rate = 2.3%, 95% CI = 2.1-2.4%). Healthcare workers, including those caring for infants and children, had seroconversion rates similar to pregnant women (summary annual rate = 2.3%, 95% CI = 1.9-2.9%). Among day-care providers, seroconversion rates ranged from 0 to 12.5% (summary annual rate = 8.5%, 95% CI = 6.1-11.6%). Parents whose child was not shedding CMV were much less likely to seroconvert (summary annual rate = 2.1%, 95% CI = 0.3-6.8%) than were parents who had a child shedding CMV (summary annual rate = 24%, 95% CI = 18-30%). Nevertheless, over the course of a year, most parents exposed to a CMV-shedding child do not become infected. Other groups with elevated risk included families with a CMV-shedding member, female minority adolescents and women attending sexually transmitted disease clinics. The relatively low rate of CMV seroconversion in most populations is encouraging for behavioural interventions and for vaccine strategies attempting to prevent infection during pregnancy.