The Rh factor is a protein that can either be present or absent on the surface of your red blood cells. It is found in all rhesus monkeys, thus the name Rh. Most people have it too, but some do not. Whether a person is Rh factor positive or negative depends on how the genes were passed down from his or her parents. If either parent is Rh positive, the baby may be Rh positive or negative. If both parents are Rh negative, the baby will also be Rh negative. All pregnant women will be tested for Rh status during routine bloodwork taken early in pregnancy.
Problems can arise if the mother is Rh-negative, and the baby is Rh-positive. Because the mother does not have this particular protein in her body, her immune system will perceive it as a foreign and possibly harmful substance. Once she is exposed to Rh-positive blood (as fetal cells move through her system during pregnancy or at the delivery), her body will make antibodies that can ‘attack’ the Rh protein. She is then said to be ‘Rh-sensitized’. In most cases, this sensitization will not happen until delivery, in a first pregnancy. It is the subsequent pregnancies that are of concern.
The antibodies from an Rh-sensitized mother can cross the placenta and enter the fetal blood stream. These antibodies then destroy some of the baby’s blood cells and cause fetal anemia, which is called Rh disease. Since the job of red blood cells is to carry oxygen around the body, Rh disease is quite serious.
If you have Rh-negative blood and do not already have antibodies for Rh-factor, you will be given an injection of Rh immunoglobulin (RhoGAM) at about 28 weeks gestation. This prevents sensitization in 99% of women so that your body will not form antibodies to the baby’s Rh factor. An Rh immunoglobulin shot may be given if you have any vaginal bleeding during your pregnancy, and will definitely be given again after delivery if your baby is found to be Rh positive.
If you are Rh-sensitized (have formed antibodies), your baby will have regular testing to assess the extent of Rh disease. Some babies do not require any treatments. If the baby is suffering from Rh disease sometimes early delivery of the baby may be recommended. In the most severe cases, if the baby is not old enough to be delivered, the baby can be treated with a blood transfusion while still in the uterus. This is very rare.