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 detecte d in 41 of those 112 cases (36.7%) where rubella virus infection of the mot her was apparent, only 7 of 141 cases (5.0%) showed evidence of fetal infec tion when maternal rubella infection was inapparent. All 184 babies born of genome-negative mothers have no congenital disorder, while 2 out of 7 geno me-positive babies have a congenital disorder (28.6%). Rubella virus was no t transmitted across the placenta when infection occurred prior to gestatio n. Transmission rate increased to a maximum level during the first trimeste r and declined to 0% until 20 weeks of gestation. Interval of viral transmi ssion from the onset of rash in the mother was about 10 days to the placent al villi and 20-30 days to the fetus. A phylogenetic tree of 61 virus isola tes suggested no difference of virulence/teratogenicity among the virus iso lates.