Anyone else RH- or O- I have a question

So with being O- during pregnancy you get a shot at 28 weeks so baby's blood doesn't mix with your or that's how my doctor explained it to me, well my doctor isn't giving me mine until 30 weeks pretty much because she's booked also not doing the glucose test until then. When did you girls get the shot, did anyone get it later ? I'm just worried because I look online and every site says they give it at 28 weeks.