when i used the HCG boosters on day 4 and 8 or another cycle day 3 and 6 it would show positive on a HPT but would continue to test until it was out of my system, or not then i knew it was a true BFP.

Im unsure why you would be having the injection on day 12 though, what dose are you on, mine was 500iu each injection.

I also had delaying af but only by a day or two, every one responds differently though. let me know how you get on and GL