If you are immune then why are you worried? Rubella is also called German measles. When we are baby we are given the MMR vaccine (Measles, Mumps, Rubella). But when we grow older, sometimes it happens that the effect of MMR vaccine fades and then our immunity to Rubella also weakens. Getting rubella during pregnancy can cause complication in the baby later. Thats why the doctors test for rubella.