in the case of pregnancy, when an rhd- mother carries an rhd+ fetus, some of the fetal red blood cells may cross the placenta into the maternal circulation, sensitising the mother to produce anti-rhd antibodies. since the mixing of fetal and maternal blood occurs mainly during separation of the placenta during delivery, the first rhd+ pregnancy rarely causes any danger to the foetus as delivery occurs before the synthesis of antibodies by the mother. however, if the mother were to conceive another rhd+ child in the future, the anti-rhd antibodies will cross the placenta to attack and lyse the red cells of the foetus, causing the aforementioned haemolytic reaction in the foetus known as haemolytic disease of the newborn. this disease is usually fatal for the foetus and hence preventive measures are conducted.