Hello In our immune system, there s a protein called an antigen, which is responsible for our blood groups (A, B, AB, O) and also there s another antigens called rhesus which is signified by (+). You have a blood group (O+) which means that you have those antigens and your body is adapted to it, but your wife is (O-), so she hasn t those antigens and her body will be sensitive to that protein in case of pregnancy and if the baby has a positive blood group and that will be responsible for producing of bodies called antibodies by your wife immune system which attacks that protein existing in the blood of the baby through the placenta and might be the cause of miscarriage. If your wife doesn t have a blood transfusion and that is her first pregnancy, that means she hadn t any contact with that antigen and the first pregnancy should be save, the risk is for the next pregnancy. But in all cases, for prophylaxis cause your wife should take an injection called Anti-D which is antibodies working by covering the antigen exists on the blood cells of your baby, so the mother immune system will not recognise them. You should ask the midwife for the injection, which should be at 28 weeks of pregnancy and repeated at 34 weeks of pregnancy. Hope I answered your question Best regards Dr. Noor Farid