Question about periods

Cat
Ok! So. Can someone help me understand this? 
Basically I want to know why the last period is counted as the start of your pregnancy, when all of us are here counting down the days from ovulation to test time/period time. Does this mean we may still get our periods this month even though we are pregnant? 
I hope this makes sense!! I mc'd last month and I'm in my tww right now. Maybe? I'm actually not sure. I'm just guessing by listening to my body and restarting to track everything. But!! If I get pregnant this cycle, I'll probably be asked when my last period was... Which was actually back in January, so I know that won't be accurate. 
Any insight or just even guesses would be welcome!