If a Rh-ve pregnant mother is having a Rh+ve foetus, then Rh antigens of the foetus are not exposed to the Rh-ve blood of the mother in the first pregnancy because the two bloods are well separated by the placenta. But during the delivery of the first Rh+ve child, some amount of foetal blood is exposed to the maternal blood. In this condition, the mother starts preparing antibodies against Rh antigens in her blood. These antibodies from the mother (Rh-ve) can leak into the blood of the foetus (Rh+ve) during next Rh+ve pregnancies and destroy the foetal RBCs.