Apologies Mum is O RhD Negative (that was a typo!!) Twin 1 = Ror Twin 2= R2r Twin-Twin transfusion usually not possible with dichorionic diamnionotic, primarily only seen with monochorionic. Just to note Twin 2 was grouped pre 1st IUT at 27 weeks when the Anti-D levels were not near as high as when Twin 1 was grouped in the neonatal period at 34+3 weeks gestation, Twin 1 Grouped as A but weak reactions against the anti-D reagents, we assumed that this could be the blocking effect- where high anti-D levels result in blocking of D antigen sites resulting in false negative or weakened RhD group. Elusion demonstrated that it was Rh D+ve IgG and IgM were coating the baby red cells , DCT strong positive (4+), IgG 4+ IgM 2+ ? Origin of IgM. On RhD antigen density Issit & Anstee quote the following: R2r has 14,000 to 16,500 D antigen sites Ror has 12,000 to 20,000 D antigen sites Initially I assumed it may have been due to the fact the twin was Ror until I discovered the no of antigen types in Ror and R2r