My limited understanding is that when the mum is O blood group and the baby is A or B, if the bloods come into contact with each other the mum develops antibodies to the babies blood. Usually the babies become jaundiced earlier and higher levels than normal jaundice.

As far as I know, there isn't a way to test for it in pregnancy (might be wrong here), I don't call seeing it tested for anyways it's usually picked up after birth when the baby develops jaundice and has a blood test for it's blood group and antibodies.

Hope that helps, not sure if it will x