If you have an Rh negative blood type you will need to get a shot of medicine called Rhogam at 28 weeks of pregnancy. When you deliver, the baby’s blood will be tested and if it is Rh+ you will need a second Rhogam shot within 72 hours of delivery. Since we don’t know the baby’s blood type before you deliver, everyone should get Rhogam at around 28 weeks. The only exception is if your blood is already sensitized. A blood test will be done prior to administration of the Rhogam to make sure your blood is not sensitized.

Everyone has a unique blood type. This is expressed with one or two letters followed by either a “+” or a “-“. Some examples of blood types are A+, B-, AB-, O+, etc. The + or – is a designation of whether or not the blood contains a certain protein called the Rh factor, this stands for Rhesus after this type of monkey as the rhesus monkey was used in some of the early experiments. The Rh protein is not an active protein, the blood works the same whether or not it contains the protein. About 90% of people have that protein and are considered Rh+, 10% of people lack the protein and are Rh-.

If a women is Rh- and has a baby that is Rh+ problems could happen. If some of the baby’s blood gets into the mother’s circulation, the body might recognize those blood cells as different. This will in essence vaccinate the mother against the Rh protein. There usually aren’t any problems for that baby but if the women gets pregnant again with an Rh- baby the mother’s immune system might attack the developing baby. Fortunately there is a treatment to keep this from happening.

Rhogam is an injection that can be given to the women to block the baby’s blood from causing the vaccine-like reaction in the women. We give the Rhogam shot at 28 weeks in all Rh- women and within 72 hours of delivery in Rh- women who give birth to Rh+ babies. In addition, if there is risk of the baby’s blood mixing with the mother’s blood earlier in pregnancy, an additional shot of Rhogam is given. Examples of this would be if there is significant bleeding in the late first or in the second trimester. When an amniocentesis is performed, an extra Rhogam injection is given.

The development of the Rhogam protocol is an interesting story. The studies were done on male prisoner “volunteers”. The Rh- males were given Rh antigen (the equivalent of a women getting Rh+ baby’s blood in her circulation). 72 hours later they were given Rhogam and it was shown to block the immune response. The reason 72 hours was chosen is that was the soonest the warden would give the researches access to the prisoners. The 72 hours is often used as a hard limit of when Rhogam must be given I find it amusing when I hear people talk in absolutes considering the origin of the 72 hour time period. The next step was to try the Rhogam on women who had given birth to an Rh+ baby but were Rh- themselves. When given within 72 hours it worked well, however about 5% of women had already been affected prior to delivery. The Rhogam lasts in the system for about 12 weeks. It was then decided to give a Rhogam shot at around 28 weeks, that way it would last until delivery when a second shot would be given. With this protocol, more then 99% of women do not get affected.