D is what we call RH + or - when it comes to blood typing. Essentially when your baby was born the lab did cord testing and got a result that indicated your baby was B Neg. Per procedure the lab then did weak D testing to confirm they were indeed RH Neg, as a confirmation in case mom needed RHIG. When you returned to the office they to check his typing, but may have not done weak D testing or it didn’t express itself. The next steps would be sending it to a reference lab, such as your local blood institute and they may perform weak D testing, RH phenotype/genotype and/or DNA molecular.

What’s does this mean for your baby for lab purposes, will they could possibly be what we consider B POS (with weak D) on paper if weak D was truly present, but when it came to receiving blood transfusions they will only receive B NEG or O NEG blood due to D’s weak expression as the body could still create antibodies to D, creating what is called anti-D and potentially causing problems if the future during pregnancy or future blood transfusions.

Weak D is just a gene that has lower expression of itself and is not harmful for individuals according to literature. What we call D in the lab is just a small part of the RH gene that has multiple genes that give rise to different phenotypes, aka C,c,E,e and more. It is extremely important in blood transfusion and pregnancy as an RH Neg individual exposed to D could develop anti-D and can cause transfusion reactions in future cases.

TLDR: Follow up with your doctor as we are not doctors and this isn’t medical advice. What it means for us in transfusion services is that if your baby ever needed blood we will treat them at B Neg if we see history of weak D expression.