Final answer:
A Rhogam injection, which prevents HDN by targeting fetal Rh-positive red blood cells, is not given at 28 weeks in the event of a rupture of membranes because this might indicate that delivery is close and could alter the treatment timing.
Step-by-step explanation:
A Rhogam injection would not be given at 28 weeks if the event of a rupture of membranes occurs. Rhogam, short for Rh immune globulin, is administered to prevent the development of Rh antibodies in an Rh-negative mother, which could lead to hemolytic disease of the newborn (HDN) if the baby is Rh-positive. This treatment is crucial during the 28th week of pregnancy and within 72 hours after birth, as it targets fetal Rh-positive red blood cells that might enter the mother's system, preventing a potentially hazardous immune response in subsequent pregnancies. A rupture of membranes can signify that delivery may be imminent, which can change the timing or necessity for the administration of RhoGAM.