Ay, ganyan talaga kapag irregular ang period, napaka-confusing! Since your ultrasound sa December showed January as your due date, I'd really suggest trusting that more than your counting, the ultrasound is more accurate especially pag irregular kayo, pero para sure ka talaga at ma-follow up properly, better ask your OB directly ano ba ang official due date nila based sa scan kasi sila ang makakagabay sa inyo ng maayos sa buong pregnancy. A few of us moms had the same worry, at what worked was just clarifying straight with our doctor para walang second-guessing throughout. Congrats by the way, and wishing you a healthy pregnancy!