However, since the fetal red blood cells usually leak during childbirth, this will not affect that fetus. But let's suppose the same woman who has been exposed to these antigen D proteins and now contains those antibodies against antigen D. Let's suppose she once again becomes pregnant with a fetus who is once again RH positive. What will happen now? Well, now we're going to have something called RH incompatibility. And what that means is because these antibodies that were produced as a result of that first pregnancy are floating around in the bloodstream of the mother and because they're small enough, they can pass across the fetal membrane into the bloodstream of that fetus. Now, how is this a problem?