A seroepidemiological study of 207 pregnant women living in Marianao, Havana City, was carried out to measure rubella, cytomegalovirus and herpes simplex anti-lgG antibodies. The cutoff level was established as the ratio between the sample's fluorescence and the positive control. The cutoff level for rubella was 0.3; for cytomegalovirus, 0.2, and 0,185 for herpes simplex. The seroconversions were considered as the increase of this ratio as related to the determination during the first trimester of pregnancy. 91.3% of the pregnant women appeared to be positive to rubella, 92.7% for cytomegalovirus, and 98.1% of the pregnant women appeared to be positive to HSV. This study underlined the need to take into consideration some important issues concerning these infections within the National Care Program for Pregnant Women.