If your baby has a different RH (+,-) than the mom, it could cause problems while the woman's pregnant for the baby if there's a chance of the blood intermingling, like during an amnio or during birth. In extreme cases, it could cause miscarriage or health problems for the baby. They give a shot at 26 weeks and after the baby is born if the baby has a different RH than the mom. It could cause problems for future pregnancies because the body attacks the fetus if it has a different RH. I think it's only 15% of the population that is negative. I read that it is common among French Canadians.