Late implantation question-please help!

Katie • 27 years old, married
I got thinking about late ovulation and implantation and would love to hear your thoughts please as I'm having a dilemma. 
​I think it's safe to assume that the later you ovulate the later your likely to implant assuming your egg is fertilised.  If this is the case and it could take x number of days to implant, what happens if your egg hasn't implanted yet and you are due AF? Does AF come regardless and just get rid of the fertilised egg? Or does the uterus have some way of knowing it's fertilised and may still try attempting implantation?! Is this a crazy question??? 
​I ask because my AF is due tomorrow at only 12dpo (28 day cycle) and in theory, I may not have had time to implant yet and I guess I'm wondering if il ever get chance if AF comes on time?  
hpt as of today (11dpo) says not pregnant. But it could just be because it's too early. 
​(Is it just me or does ttc / tww mess with your mind on the meaning of life!!) 
​Any thoughts?! And when should I test if AF doesn't come? As it's BFN as of today!