erythroblastosis fetalis

[ainda]

The mother CAN NOT be Rh + and the baby Rh -, simply because the Rh neg allel is recessive!

[MrMistery]

is the mother is Dd(Rh+) and the father is both Dd(Rh+) then 1/4 of children will be dd(Rh-). Do a punner square

[genovese]

Even if an Rh° baby developed antibodies to his Rh+ mother at the time of birth there would not be enough time to produce a clinical effect in the mother. This would be the same with subsequent pregnancies, so the Rh+ mother would never develop an Rh incompatibility problem.

The Rh° babies will have been sensitized to Rh+ blood and might have a stronger reaction to it if transfused with it.

[Darby]

The antibody response is to the rh markers. If you are rh-negative, those markers are "foreign," so once exposed you produce antibodies to them. If you are rh-positive, exposure to rh-negative blood doesn't give you something to make antibodies to.

I have to disagree about blood mixing, though. The tearing free of the placenta mixes exposes the mother to the fetus' blood pretty reliably. There are also many instances of what would otherwise be minor exposures during pregnancy, as well as cases of maternal rejection of rh-positive fetuses in a first pregnancy.