There is a short story and a long story in answer to your question. The short story is that this is a routine matter that is handled as a matter of course at the obstetrician's office.

The long story is as follows. The Rh factor is actually a protein that appears on the surface of red blood cells. Some people have this protein others do not. The protein itself is of no signficance--it is merely a marker that helps the body identify self from non-self. About 85% of the population does carry this protein. Those that do not have it are designated as Rh negative. Rh negative women who carry Rh positive babies can develop an immune response to subsequent Rh positive fetuses. While being Rh negative is never a concern for the first pregnancy, the risk of an immune response to Rh positive fetuses rises with subsequent pregnancies. Such an immune response would involve maternal antibodies crossing the placenta and destroying fetal red blood cells, resulting in fetal illness or death. A grim story except for the fact that a vaccine has been developed (one popular trade name is Rhogam) that has been shown to almost completely prevent potential immune responses against future babies when given in the first few days after birth. Common practice now has been expanded to include Rh vaccination at 28 weeks as well since a very few mothers have been known to develop immune responses in the third trimester.

Rh incompatibility was a signficant issue several decades ago but has been almost completely eliminated by modern obstetrical practice.

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