Rh immunoglobulin is a medication used to prevent Rh incompatibility during pregnancy. It is given to Rh-negative mothers who may be carrying an Rh-positive baby. If the baby's Rh-positive blood mixes with the mother's Rh-negative blood, it can lead to serious health issues for the baby, such as hemolytic disease. The injection works by providing the mother with antibodies that destroy any Rh-positive blood cells before her immune system can react to them. This treatment is typically administered around the 28th week of pregnancy and again after delivery if the baby is Rh-positive.