so confused...

okay not sure really how to explain this but...

My period was on the 3-6th december and i ovulated on the 14th ( did opk) even though all the apps said the 20th. If i was to start from my period i’d be 2 weeks and a few days but if i start from my ovulation date i’d be 3 weeks. what would you start from if you were calculating weeks of pregnancy? i only want to know so i can figure out if i’m testing too early.