Cytomegalovirus (CMV) is the leading cause of viral congenital infections. In children, the consequences may be severe, especially in case of maternal primary infection during pregnancy. A prospective study was carried out in the department of Isère, in 1,018 pregnant women, in order to establish the seroprevalence of CMV, the frequency of primary infections during pregnancy and the associated risk factors. The overall seroprevalence was 51.5%; it increased significantly with age, parity, and low socioeconomic status. It was higher in women born in the South of France (51.6%) than in those born in the North (37.4%). Among a total of 878 women with serological follow-up, 7 primary infection cases (0.8%) were observed. Seventeen women (1.9%) presented border IgM values in the first serum, and these values were not related to recent infection. Extrapolation of the results to the whole department of Isère, suggests that each year about 100 pregnant women would be concerned by CMV primary infection, with 2 or 3 cases of death or severe sequelae in children. In light of these results, the interest of serological screening is discussed.