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There are four major blood types: A, B, O, and AB. The types are based on substances on the surface of the blood cells. Another blood type is called Rh. Rh factor is a protein on red blood cells. Most people are Rh-positive; they have Rh factor. Rh-negative people don't have it. Rh factor is inherited though genes.
When you're pregnant, blood from your baby can cross into your bloodstream, especially during delivery. If you're Rh-negative and your baby is Rh-positive, your body will react to the baby's blood as a foreign substance. It will create antibodies (proteins) against the baby's blood. These antibodies usually don't cause problems during a first pregnancy.(Read more)