that's not strictly true i am neg but both mum and dad are pos so googled it and founf this (i have copied and pasted cos i'm crap at doing links )
There are four possible blood groups: A, B, AB and O. Everyone belongs to one of these four groups and except in very rare instances, it doesn't matter if you are a different blood group to the father of your baby. What does matter is the rhesus factor - whether or not you are rhesus-positive (RhD-positive) or rhesus-negative (RhD-negative). People who are RhD-positive have a protein on their red blood cells called D antigen, RhD-negative people do not have this. If you are RhD-negative and are carrying a RhD-positive baby, there is a risk that if yours and the baby's blood mix (which could happen during pregnancy or the birth) your immune system will produce antibodies against your baby's blood, which will cause anaemia and other problems in this baby or in future pregnancies.
We inherit the rhesus factor from our parents, and approximately 85% of the Caucasian population has RhD- positive blood, 94% of Africans and 90% of Asians. The reason most people are RhD-positive is due to the fact that we each have two copies of the rhesus gene (one inherited from each parent) and these genes are either positive or negative, with the positive one being dominant and the negative one recessive. This means that if you have one positive gene and one negative, the positive one always wins and you will be RHD-positive. If you have two positive genes you will also be positive. Only if both genes are negative will you have RhD-negative blood.
If both parents are RhD-negative, then there is no chance that any of their babies could be RhD-positive, since neither parent possesses a positive gene to pass on. If both parents have a positive and a negative gene (so they will both be RhD-positive), the chances are that one or both parent will pass on a positive gene to their baby, so the baby will also be rhesus-positive. It is possible for two rhesus-positive parents to produce a RhD-negative baby, so if your parents are both positive and you are wondering how you can possibly be negative, here is the answer! If both your parents carry a positive and a negative gene, they will both be RhD-positive because the positive one is the dominant one. But if both parents pass on the negative gene to the baby, the baby is bound to be RhD-negative as it has not received any RhD-positive genes from either parent.
Blood tests to find out your blood group and rhesus status are offered early in pregnancy because complications arising from the rhesus factor are preventable. Your midwife will give you more information about this. Your partner may not know his blood group and rhesus status unless he is a blood donor or has had this tested for some medical reason in the past. It is not usual to offer partners routine testing to find out this information, though you might consider asking if this could be done.