I am a former lab technician and also a mother with Negative Rh factor. I'll try to explain it as best as possible. All people have multiple antigens in their blood. Rh factor is merely a term used to identify one of those antigens which is actually called D/d. Individuals who have DD or Dd are considered Rh +. Individuals who recessive, dd, are Rh -. This is where it gets a bit complicated. A mother who is Rh -, or dd, and a father who is Rh -, or dd, can ONLY pass on recessive genes to their offspring. Therefore their children will 100% of the time be Rh -. However, if the mother is Rh -, or dd, and the father is Rh +, either DD or Dd, there is NO way to determine which gene he will pass on. If the father carries two dominate genes, all of the children will be Rh +. If the father carries one of each, there is a 50% chance that the child will also be Rh+. If the mother and child have two different Rh factors, then the mother's blood can cross through the placenta and attack the baby's blood causing serious blood problems. Most often than not, the first pregnancy is not affected. If not treated however, the antigens remain and they will attack the growing baby from the next pregnancy usually causing a miscarriage. RhoGam is a serum which causes any antibodies that have built up to be collected and flushed out of the mothers blood stream. The injection is ONLY necessary if the father of the child is Rh +. My husband and I share the same blood type, A -, so I refuse to take the injection. Every pregnancy someone gives me hoo-hah about it and I am required to sign a medical release. I am not a fan of injections, but from personal and professional experience this is an incredible scientific discovery. It has saved countless numbers of children from death and serious blood disorders.