Congenital rubella synrome (CRS) can occur in a developing fetus of a pregnant woman who has contracted rubella during her first trimester. If infection occurs 0-28 days before conception, there is a 43% chance the infant will be affected. If the infection occurs 0-12 weeks after conception, there is a 51% chance the infant will be affected. If the infection occurs 13-26 weeks after conception there is a 23% chance the infant will be affected. Infants are not generally affected if rubella is contracted during the third trimester, or 26-40 weeks after conception. Problems rarely occur when rubella is contracted by the mother after 20 weeks of gestation. It was discovered in 1941 by Australian Norman McAllister Gregg (1892-1966).