Ovulation is what causes your period. You usually ovulate about 12-16 days BEFORE your period is due. If you ovulated on day 15 then you will either get pregnant or get your period about 2 weeks after that date.
I dont know if this is true or not... I am pretty posiitive i ovulated my last 3 cycles before my pregnancy as late as CD 19....
The cycle before my BFP I am pretty sure I ovulated CD18.
You might be more at risk for chemical pregnancies because if your luteal phase isnt long enough then it wont implant properly and the hormone production to stop your period wont happen..
I should add though, even though I ovulated later than 15, my luteal phase was still long enough because my cycles were longer than 28 days