BD at the end of the cycle?

Hi,
This sounds like such a silly question but I am so new to this. What if you BD (baby dance!) right at the end of the cycle, right before your period's due? I know it's a low chance that you'll conceive at this point anyway but what if you did? Does AF come that month as planned but not next? Or would AF hold off straight away if you were pregnant? 
Any help gratefully received! Xx