Actually it's a lot more complicated than that. It's all to do with recessive genes etc. For example, I am B+ and my OH is A+ but Kaya is O+. What you do have to know though is that if your OH is Negative but the baby is Positive she will need a resus injection after she gives birth or it could seriously affect her future chances of conceving. However, her doctor/midwive should explain this to her.