The hospital blood bank now will only transfuse o- or o+ for preemies. they will not use the matched blood. why?

Mom made antibodies?: The red blood cells should be group o or other ABO group-compatible with the infant and mother, rh-negative or rh-identical with the infant, and lack red blood cell antigens to which the mother has made alloantibodies. So, if mom made antibodies against a or b, some of those antibodies may be in the baby's blood plasma (and would react with transfused a, b, or ab blood cells).