Rhesus factors (Rh) are factors present on the surface of red blood cells. If these factors are present on a person's red blood cells they are said to be "Rhesus (Rh) positive". If they are not present on the person's blood cells, then the person is said to be "Rhesus negative". Rhesus incompatibility between a future mother and her foetus may cause serious problems. If a Rhesus negative mother has a Rhesus positive foetus, while no problems generally occur during the first pregnancy, during a subsequent pregnancy the mother's body can develop antibodies against the Rhesus positive blood. This then leads to what is called "haemolytic disease of the new-born".