It's because I'm Rh- and thus can make antibodies against Rh+ blood should I come into contact with it. IF the baby is Rh+ and IF the mother/baby barrier breaks down and IF the next baby is Rh+, then I could reject the next pregnancy. BUT the injection has to be given within 3 days of the blood mixing, so giving me a set date means exactly diddly squat because what happens if there's a problem a week before the routine injection? I know, I ask for an emergancy one, but why bother with the routine ones? It seems really stupid to me.