As a practicing obstetrician, I'm happy to report the good news—today almost all pregnancies complicated by Rh-negative concerns do well. When you are pregnant, the old adage is true: good fences do make good neighbors. Your blood and your developing baby’s blood remain separate, and the membrane that separates them maintains a peaceful coexistence until delivery. Here's how your body is very well designed to protect you and your baby.
How The Placenta Protects
The placenta is the membrane attached to your uterus that separates your blood from your baby’s blood. The placenta lets some things through but not others, and there is a size limitation on what can pass freely between baby and you. So smaller things like oxygen and carbon dioxide molecules have no trouble, but bigger, bulkier structures such as bacteria or red blood cells are blocked. In this way, oxygen can pass through this barrier from your red blood cells to your baby’s red blood cells without the two types of cells entering the others’ domains and setting off a huge immunological response, or allergic reaction. It works well, but there are some holes in the system.
What Invading Antigens Do
Your entire immune system depends on its ability to fight foreigners, called antigens. Your body makes antibodies when an antigen gets into your system. The antibodies you make for you are small, though—small enough to slip through the placenta. This is a good thing, because it allows your baby to be protected at birth with a host of your antibodies with which to fight off infection during the first few months of life.
Invading antigens, however, are usually attached to bigger structures, like red blood cells. These are too bulky to pass between your baby and you. Your baby’s red blood cells, usually different from the type you have—thanks to that big foreign body, your husband—can’t get to you, and neither can the antigens they carry. The separation of your and your baby’s circulations means there is no mixing of your two different blood types. You aren’t exposed to your baby’s red blood cells and you won’t generate antibodies to your baby’s blood. This is good because if you did, these antibodies made to fight your baby’s red blood cells would be small enough to get back to your baby. Inheriting your antibodies to fight infections is one thing, but your baby inheriting antibodies to herself is another thing altogether—a dangerous thing.
There’s a small detail I haven’t mentioned yet—that little process called giving birth. During your baby's birth, the placenta barrier is broken up, allowing a brief opportunity for your red blood cells and your baby’s to mix. The importance of this brief mixing doesn’t necessarily apply to the baby just delivered, but to future babies, as I’ll explain later.
Blood Types and Antigens
People identify their blood types as A, B, AB, or O. ABO is the main blood cell antigen system. But there’s also the CDE system. Each of these systems refers to a type of red blood cell antigen that identifies itself as an enemy or a friend as reliably as a serviceman’s uniform. So, blood types are called A, B, or AB, because they have the A-, B-, or both the A- and B-antigens.
Different blood types just don’t get along very well. That's because antigens will generate an allergic reaction in a person given a different blood type. In the most severe cases, getting blood that doesn't match yours could cause an allergic reaction leading to death. While these instances are rare, A, B, or O reactions between you and your baby occur often when the two blood streams mix briefly at birth. But this response is usually mild.
The Rh Factor and Blood Incompatibility
In pregnancy, the Rh hoopla centers around the “D” antigen of the CDE system. This antigen is called the Rhesus D because the initial research was done with Rhesus monkeys. If you have D-antigen-type red blood cells, you’re “Rh-positive.” If you don’t, you’re “Rh-negative.” The reaction to D—the Rh antigen—can be very serious.
If you and your baby are Rh-negative, there’s no problem, since you both have the same Rh type. If you’re Rh-negative and your baby is Rh-positive (thanks to your husband's genes), that’s fine until your blood mixes with your baby’s blood a bit during placental separation at birth. At that critical point, fetal blood cells can accidentally combine with your system, and you make antibodies to fight them. Your immune system is successful in vanquishing these stray fetal blood cells in your circulation. This is of no consequence, because once they’ve gotten rid of the baby’s blood cells in your system, they have no other job. And they can’t filter back through to your baby’s blood, because delivery has already taken place. The antibodies you made just remain in your circulation waiting. For what? For your next pregnancy. Herein lies the problem.