Maternal rubella during the first trimester of pregnancy probably causes fetal infection in the majority of cases. Fetal infection, however, does not invariably cause major defects. The incidence of the latter has been estimated in various prospective surveys as 10 to 45 per cent. After the first trimester, and particularly during the fourth month of pregnancy, the risk of serious fetal damage declines sharply.In this report, the authors describe a serological study of 304 infants whose mothers had had confirmed rubella at various times after the 12th week of pregnancy. They have tried to measure the frequency of fetal infection in two ways. First, they have used immunofluorescence (IF) and radioimmunoassay (RIA) to detect specific IgM antibody soon after birth. Second, they have used the same methods to detect persistent IgG antibody in whole serum after the age of 8 months, when maternal antibody should have disappeared. The infants formed part of prospective study and were not selected retrospectively because of abnormalities. Almost all of them were normal and healthy at birth.In order to measure the prevalence of rubella IgG antibody in infants with no known maternal history of rubella during pregnancy, the authors tested sera from 200 children, aged between 10 months and 3 years, by IF. These children had an age distribution similar to those in the main study group.The authors compared IF and RIA by using both methods to test sera from 194 infants for IgM antibody. In 55 of 58 sera, the fractions of which were positive by IF (titers 2 to 512), RIA was also positive (titers 400 to 70,000). Three sera were IF-positive but RIA-negative, and one was IF-negative but RIA-positive. These four sera with discrepant results came from children who subsequently had persistent IgG antibody at ages between 10 and 13 months and who were, therefore, probably infectedin utero. The remaining 135 sera were negative by both methods. Agreement was sufficiently good for the investigators to accept positive results obtained by either method and to conclude that 59 sera contained specific IgM antibody. They tested sera from 72 other infants by either IF or RIA and found rubella IgM in 18. In all, they detected IgM antibody in 77 of 266 infants (29 per cent).Ninety-four of 190 sera (49 per cent), taken at ages between 8 months and 3 years, were positive when tested for IgG antibody by IF. When 57 of these were tested by RIA, 54 were positive (100 or greater), and three were negative. Ninety-six sera were negative when tested for IgG antibody, 54 by both methods and 42 by IF alone. No serum was IF-negative but RIA-positive. Agreement between the two methods was sufficiently good for the investigators to conclude that all the IF-positive sera contained IgG antibody.All infants with IgM antibody who were followed up were found to have persistent IgG. A minimum figure for the fetal infection rate can therefore be obtained from the proportion of infants who were tested for IgM and were found to be positive. For the whole period, this figure was 77 of 266 (29 per cent).
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