PCOS and 18w5d!!!

Hello Ladies! So I found out at work on Nov 21st that I was pregnant!!!  I never have periods and I mean NEV-ER! I had an ultrasound on Nov 25th that dated me at 10w3d DD 6/20 and on Dec7th at 12w5d DD 6/16, is there anyone else who has PCOS who isn't quite sure when they conceived or how far along they are?? I feel like my ultrasound may be putting me a little further than I may actually be