The theory is that lh and hcg are like twin hormones. An opk can pick up hcg but a hpt cant pick up lh. Its certainly not exact as many women experience an lh surge b4 af is due (I did one cycle - got all excited for nothing). Its more just a possibility if you see a line getting darker on opks then you MIGHT end up seeing a bfp or you MIGHT not. I'm only poas because I'm an addict lol and have no need to keep them :) and its way to early to give me any indication lol but just out of curiosity I thought I'd test up to 6dpo just to see what happens :)