Most of us are O, some are AB, some others are either A or B. Blood of all human beings find place in one of these categories, namely, O, AB, A and B. Rhesus factor present in the blood decides whether it is positive or negative. Thus, the final identification of your blood type is that of the particular blood group together with Rh factor. That is how blood groups get described as O positive, B negative etc.
It is absolutely normal for an individual to be Rh–negative. The problem pops up when an Rh-negative woman becomes pregnant with the child of an Rh-positive man. In such a case, irrespective of their ABO grouping, they might land in trouble if adequate precautionary methods are not taken. If the baby happens to fall into Rh-negative category, there will absolutely no trouble. But, if it is an Rh-positive baby, complications are likely.
In all pregnancies, some blood of the foetus will mix up with that of its mother, especially during the time of the delivery. If the baby is Rh-positive and the mother is Rh-negative, mother’s body reacts by creating antibodies. This is because; the Rh-positive blood cells are foreign particles that should be fought against, as far as her body is concerned. These antibodies might cross over to the baby through the placenta. In such a case, the baby’s blood cells get battered. The first baby will somehow escape, as the mother’s body require a period of few months to build these antibodies. In case of subsequent pregnancies, danger factor is high since the mother’s body is already equipped.
That is exactly the reason why gynaecologists double-check your blood group during pre-natal tests. If found to be Rh-negative, the mother will be given injections during the pregnant months and also immediately after the delivery in order to prevent the development of antibodies.