I need help understanding a strange phenomenon.  I have a D negative Mom by gel testing that has delivered  a D positive (3+) infant by gel testing.  A maternal post delivery sample was used for an FMH screen.  The FMH screen was macroscopically positive 1+^w.   The Mom's post delivery sample was tested for weak D and was found to be 1+^w.  The Mom's pre delivery specimen was tested for weak D and it was negative.  The sample was tested for FMH with a KB stain an no fetal cells were seen.  I was expecting the KB stain to reveal a large number of fetal cells. 

What were the blood types of mom and baby?

What were the blood types of mom and baby?

Mom is O Neg and baby is O Pos.

First, you have a discrepancy between the Mom's Rh type on the pre- vs. post-delivery specimen. That needs to be resolved just as you would need to resolve an ABO discrepancy.

I would suggest that a new specimen be collected from the Mom and tested.

If the new specimen's Rh type agrees with the pre- specimen, then it would indicate there was a problem with your post specimen either misidentification or contamination. Repeat the rosette test on the newly collected post specimen.

If the new specimen's Rh type agrees with the original post- specimen then you have your answer that the rosette test is false positive due to the Mom having a weak expression of D which interferes with rosette testing. You are not detecting Rh + fetal cells, instead you are detecting Rh + (weak) maternal cells which would explain why the rosette test is positive but the KB stain is negative. You would also then need to follow up as to the pre- sample and whether it was misidentified at collection, etc.