Rh disease occurs during pregnancy. It happens when the Rh factors in the mom’s and baby’s blood don’t match. It may also happen if the mom and baby have different blood types.

What causes Rh disease?

Each person has a blood type (O, A, B, or AB). Everyone also has an Rh factor (positive or negative).

The Rh factor is a protein on the covering of the red blood cells. If the Rh factor protein is on the cells, the person is Rh positive. If there’s no Rh factor protein, the person is Rh negative.

A baby may have the blood type and Rh factor of either parent, or a combination of both parents.

There can be a problem when an Rh negative mother has a baby with an Rh positive father. If the baby's Rh factor is positive, like his or her father's, this can be an issue if the baby's red blood cells cross to the Rh negative mother. When this happens, the mom becomes sensitized to Rh positive blood.

This often happens at birth when the placenta breaks away. But it may also happen any time the mom’s and baby's blood cells mix. This can occur during a miscarriage or fall. It may also happen during a prenatal test. These can include amniocentesis or chorionic villus sampling. These tests use a needle to take a sample of tissue. They may cause bleeding.

The Rh negative mom’s immune system sees the baby's Rh positive red blood cells as foreign. Your immune system responds by making antibodies to fight and destroy these foreign cells. Your immune system stores these antibodies in case these foreign cells come back again. This can happen in a future pregnancy. You are now Rh sensitized.

Rh sensitization normally isn’t a problem with a first pregnancy. Most issues occur in future pregnancies with another Rh positive baby. During that pregnancy, your antibodies cross the placenta to fight the Rh positive cells in your baby's body. As the antibodies destroy the cells, your baby gets sick. Your baby could have jaundice, heart failure, and enlarged organs.