The RH factor...

I am now 16 weeks pregnant. The baby is the size of an avocado. The baby is going to have a big old growth spurt this week! Yeah!
I went to my doctor's appointment yesterday and things are looking good. We heard the heartbeat again. It was 160 beats per minute. I felt my uterus above my pelvic bone...how cool is that!?!
I was told that I am Rh negative. This is in regards to my blood. I had never heard of this before and the doctor explained it to me. Here is what I've gathered. Only about 15% of Caucasians are Rh negative. Depending on the father's Rh factor..which I am not sure what his is...my baby can either by Rh negative or positive. If the baby is Rh positive, our blood is incompatible. Now, most of the time the baby's blood doesn't cross the placenta over into my blood. This usually only happens upon birth. If the baby's blood does mix with mine, my blood will become "sensitized". My body will start to build up antibodies against the baby's blood and look at is as a foreign invader. So...to make a long story short, they are going to test my blood at 28 weeks to see if my blood has been sensitized. If it hasn't, I'm good to go until birth. If it has, then I will have to get a shot of Rhogam (its a shot of antibodies that will kill off the rh positive blood in my system). I will also have to get a shot at birth. The doctor, and other women who are Rh negative, have assured me that this is routine and the baby is not at risk. I'm a little nervous about it, but overall I feel confident that all will be well.
Here is an interesting update though....my maternal grandfather was Rh negative. My mother is Rh positive. My mother, after hearing this story, stated that she believed I was Rh positive at birth. She found my baby book and low and behold, the doctor did write on a slip that I was Rh positive. Unless I am some sort of alien that can change Rh factors or unless I am a chimera (for all you non nerds out there that is someone with two DNA strands going through them), I'm going to guess that my Rh factor did not change. I have a call into the doctor's office to get retested.
So....I'm good, baby is good. I may be an alien and my baby may be a foreign invader in my body. I have to make jokes or I will start looking up things on the Internet and diagnose myself.
Ill keep you all updated on my Rh status.