I've heard of this test being done in the hossie I used to work at, but it is not to test the bub's blood group. If mum's antibodies test postive, they are tested very regularly and if they are found to be increasing, they like to take a sample of the amniotic fluid to test bub's bilirubin levels. If neccessary they will then take a sample of blood from the umbilical cord to test the heamoglobin (as anti-D antibodies destroy bub's red blood cells), and if it is low enough they will transfuse bub. I have looked after babies that have had this done multiple times during pregnancy. But it is not common.