The rh factor is an antigen that can cause problems in some women who bear more than one offspring.

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There are 8 major types of blood; the RH positive types, which are A, B, AB and O and the RH negative types, which are A-, B-, AB- and O-.
The RH negative blood types are very rare; in USA for instance only 15% of the population are RH negative.
When a woman with RH negative blood type is pregnant by a RH positive husband, the baby will have RH positive blood type. When the pregnant woman is exposed to her RH positive baby's blood her body may become sensitized and begin the production of antibodies that will destroy the strange blood cells.
During the first pregnancy, the mother is only exposed to her baby's blood during birth. This exposure will make her body to produce antibodies that are specifically meant to fight the baby's blood cells which are consider as foreign bodies.
Although the antibodies are not able to fight during the first exposure, they will sit and wait until the foreign cells appear again and then they will attack. Which means that if the woman get pregnant again, the antibodies will attack the new fetus and destroy it, which will lead to miscarriage.