Rubella virus genome diagnosis during pregnancy and mechanism of congenital rubella.

Fetal infection with rubella virus was diagnosed based on the detection of viral genome in the fetus-derived tissues. While viral genomes were detected in 41 of those 112 cases (36.7%) where rubella virus infection of the mother was apparent, only 7 of 141 cases (5. 0%) showed evidence of fetal infection when maternal rubella infection was inapparent. All 184 babies born of genome-negative mothers have no congenital disorder, while 2 out of 7 genome-positive babies have a congenital disorder (28.6%). Rubella virus was not transmitted across the placenta when infection occurred prior to gestation. Transmission rate increased to a maximum level during the first trimester and declined to 0% until 20 weeks of gestation. Interval of viral transmission from the onset of rash in the mother was about 10 days to the placental villi and 20-30 days to the fetus. A phylogenetic tree of 61 virus isolates suggested no difference of virulence/teratogenicity among the virus isolates.