ObjectiveTo determine whether the rate of human cytomegalovirus transmission in utero is related to the gestational age at the time of maternal infection.MethodsOne hundred twenty-three pregnant women followed in our units between 1988 and 1998 were studied retrospectively. Each had developed a primary infection with cytomegalovirus evidenced by a seroconversion, confirmed by specific enzyme immunoassays. Infants were diagnosed by urine culture.ResultsRegardless of gestational age at the time of maternal cytomegalovirus seroconversion, the mean rate of intrauterine transmission was 57.5%. There was a statistically significant difference between early seroconversion (during the first trimester) and late seroconversion (during the third trimester) (36.0% versus 77.6%;P< .001). The risk of transmission calculated for seroconversion during the second trimester was intermediate (44.9%).ConclusionA statistically significant difference in the rate of intrauterine cytomegalovirus transmission was observed according to the duration of pregnancy at which primary infection occurred. The rate of transmission increased with gestational age.