If the mother is Rh negative, her system will not tolerate the presence of Rh positive cells. Her immune system treats the Rh positive fetal cells as if they were a foreign protein or substance and makes antibodies against the fetal blood cells. These anti-Rh positive antibodies move through the placenta into the fetus where they destroy the fetus' circulating red blood cells. Rh incompatibility may cause symptoms ranging from very mild to death of the newborn. The jaundice of Rh incompatibility, measured by the level of bilirubin in the infant's bloodstream, may range from mild to dangerously high levels.
Hydrops fetalis often results in death of the infant shortly before or after delivery.
Kernicterus develops in extremely jaundiced infants, especially those with severe Rh incompatibility. Later a high-pitched shrill cry may develop along with unusual posturing, a bulging fontanel, and seizures . Infants may die suddenly of kernicterus. If they survive they later develop decreased muscle tone , movement disorders, high-pitched hearing loss , seizures, and decreased mental ability. Rh incompatibility develops only when the mother is Rh negative and the infant is Rh positive.