Elizabeth is 20 weeks pregnant with her second child. Her blood type is AB negative. Her obstetrician is concerned about the possibility of a reaction between Elizabeth's blood and the baby's blood because of the father's blood type. The obstetrician prescribes RhoGAM for Elizabeth. RhoGAM contains immunoglobulins that prevent the production of certain antibodies. What condition does the obstetrician fear may be a complication with this pregnancy in the event that RhoGAM is not given?