More people than not have the RH factor and more people are O. I happen to be A- and my husband was B- so the kids are AB-. If my husband would have had the RH factor, I would have had to had the gamaglobulin shot after the first baby. When the first baby is born, if you do not have the RH factor, but the father does, your antibodies haven't had a chance to build up and "realize" that something foreign is inside, so nothing happens. But after the first child, when the placenta breaks, they kick in and respond and build so that when the next child is inside, the antibodies attack and cause something called erythroblastosis fetalis. It's HORRIBLE. So, they give this shot to tell the immune system to chill out, that it doesn't need to over react. (a symplistic explanation)
So, it is important to know your blood type if you're pregnant the second time, which, if under a doctor's care, they would always do.