Today I got a Rhogam (Rho(D) Immune Globulin) injection to help prevent my body from making antibodies to the baby's blood-type. (Basically to keep my body from destroying the baby's blood.)
There are lots of things to consider when looking at compatibility of blood types, but generally the most important are the ABO blood type, and the Rh factor. Although there are quite a few Rh negative alleles (gene forms) in the population, it is recessive, so most people are phenotypically Rh positive (the positive allele is dominant over the negative allele).
We all have two copies of each gene, one copy from mom and one from dad. With recessive and dominant alleles, here is how the phenotype (what we observe) will be between different possibilities of R (the positive, dominant Rh allele), and r (the negative, recessive Rh allele):
Observed Allele1 Allele2
Negative r r
Positive R R
Positive R r
Positive r R
So, even though three of the possibilities above include at least one recessive (r) allele, only one person is observed as being a negative Rh factor, because the little r is always masked out by the big R.
I'm Rh negative, and Scott is Rh positive, so there's a pretty good chance the baby will be Rh positive as well. There is a 50:50 chance it will be positive if Scott is heterozygous at the Rh allele (that is, he has one positive and one negative gene form), because to be negative, I must have two negative alleles. If he is heterozygous (Rr), then the baby could be:
Rr OR rr
with equal likelihood.
However, if he is homozygous (has two dominant copies, RR), then the baby can only get a "R" from him and a "r" from be, so it will be "Rr" meaning it will have a postitive Rh factor with a probability of 100%, and my body could recognize it as something foreign and try to destroy the positive Rh factor blood cells.
What that all boils down to is that it is best to treat for the potential problem (one injection now, one after the baby is born), to prevent the risk of future complications.