There are essentially four types of major blood groups in humans - A, B, AB and O. The Rhesus factor is a substance in blood by which human blood may be divided by its presence into (Rh +ve) or (Rh -ve) groups. The final identification of a blood group is thus O +ve, B -ve etc.
There is some misconception that the same blood group in both partners may be detrimental. This is not true. The actual problem may occur when the mother is Rh -ve and the husband is Rh +ve irrespective of their ABO grouping.
The baby of such a union could be Rh -ve or Rh +ve. For an Rh -ve baby there will be no problems and no precautions need to be taken. For an Rh +ve baby on the other hand complications of varying severity may take place.
Mixing of some blood of the baby and mother occurs throughout every pregnancy but more so at the time of delivery. The mixing of Rh +ve blood (from the baby) in a Rh -ve mother causes the mother to build up some negative factors (antibodies) in her blood over time against the Rh +ve blood cells. These negative factors may then cross over to the baby through the placenta (afterbirth) and destroy the blood cells of the baby. Usually the first pregnancy is spared, as a few months are needed for the negative factors to be built in the mother's body. In the subsequent pregnancies Rh +ve babies are likely to be affected by the antibodies of the mother.
Tests may be done to estimate the amount of negative factors in the mothers' blood. (Indirect Coombs' test). This gives us an idea of the chances the baby may be affected. If the chances of the baby being affected are high - serial tests of amniotic fluid or blood directly from the baby in the womb are done. If the condition is mild, early delivery and treatment of the baby is done after birth. If severe, blood transfusions for the baby need to be carried out in the womb. These procedures are done in selected places by specialists and carry a high complication rate.
Injections are given both during pregnancy and soon after delivery to prevent an Rh -ve mother carrying an Rh +ve baby from developing the negative factors. The same injection needs to be given after a miscarriage to neutralize the mixing of blood which occurs in those cases.
Treatment as explained above essentially depends on the severity of the condition. For severely affected babies, the chance of dying is high. Precautions include early identification of the high-risk pregnancies by checking blood groups with Rh factor for all pregnant women.