At what point does your period stop?

Rose 🌹 • 18 months down, but finally pregnant with Baby no. 1!
I'm 2 days late now and 12 dpo. I'm NEVER late but all Hpts have been negative so far. 
At what point does your body know to stop your period? 
Is it possible my egg was fertilised and is just being slow to get to the uterus and implant? But it's still fertilised so no period?