this is my answer Rh (D) immune globulin (RhoGAM) is given only if antibody formation has not occured. a negative Coomb's test confirms that antibodies have not been formed. the shot is als recommended within 72 hours after birth, miscarriage, abortion, ectopic pregnancy, or amniocenteses. Rh - immune globulin contains antibodies to the Ph factor. these antibodies seek out the baby's Rh+ blood in the mother's bloodstream, attach to it, and destroy it before the mother can make antibodies of her own. Rh-immune globulin's protective effect last for about 12 weeks, so the shot will be given again during future pregnancies.