My husband and I are both RH Neg blood types, so when I was pregnant I didn't have anti D. However, I am a weak RH. Somehow I got a positive baby. Apparently we each carry 2 genes and that is how I came to have a positive baby. I was once typed as positive when I gave blood, but I was typed several times after that as I was confused. Each time I was Neg. So it can happen, rare though.