I work in a lab and we test pregnant women to determine their blood type before they deliver.
Rh factor is a genetic trait, just like eye color or hair color. Your husband is Rh positive, meaning he has the Rh factor on his red blood cells. You are Rh negative, so your red cells lack this factor.
If your baby inherits the Rh factor from your husband, she will be Rh positive. (The same way she would inherit blue eyes or blonde hair). Your body will "see" the Rh factor in her blood and treat it as a foreign object. Your body will begin making antibodies to the Rh factor (antibodies are the little "soldiers" that attack foreign things and keep your body healthy).
These antibody "soldiers" probably won't harm your first baby because they are just now forming and haven't gained strength yet. BUT - here's the important part - when you get pregnant with another Rh positive baby, those antibodies immediately spring into action and start attacking those foreign invaders (your baby's blood cells). This can cause your baby to become low on blood while it's still in the womb. Not good.
However, there is an easy treatment for this. The drug Rhogam
is given to all Rh negative mothers who have Rh positive partners. It prevents the production of anti-Rh in the mother, and ensures that future babies will be safe. Your Dr. will tell you when this is needed.
Sorry for the long answer. That's the condensed version!