If an Rh negative woman is pregnant with an Rh positive fetus, her body will produce antibodies against the fetus's blood, causing a disease known as Rh disease

Rh-induced hemolytic disease of the newborn

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Heamolytic disease of the newborn

or Erythroblastosis Fetalis

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If you are RH- and your baby RH+, and your blood mixes with your baby's, it could become sensitized. Your body could begin producing antibodies to the RH+ blood. If these antibodies get to an unborn baby, they can begin killing off the baby's red blood cells. This is called RH disease. It is rare for it to affect a first baby, but if the mother becomes sensitized and produces antibodies to RH+ blood, it could cause problems for future babies. RH disease is very serious; however, recent medical advances allow 90% of babies with RH disease to survive (March of Dimes).