Well, the only reason could be their doctors didn't recommend them to get one in the first place because they have: - Already been sensitized, meaning there's now Rh antibodies in their blood - Rh-positive blood type - Allergies to immunoglobulin - Have hemolytic anemia - Have had vaccines recently (RhoGAM may reduce their effectiveness). It's also best if you don't get any vaccines about three months after receiving RhoGAM. You may also skip a RhoGAM shot if you're Rh-negative and your partner's blood type is also Rh-negative because it would be impossible for your baby to have an Rh-positive blood type. So there's no chance that you both (mom and baby) will develop Rh incompatibility.