If 15 Dec is your first day of your last menstrual period then doc will count from that date. So as of last week (between 19-25 Apr) you’re probably about 18 weeks pregnant give or take.
Science says you are actually “2 weeks pregnant” before conception. Watch YouTube videos and you’ll understand. At first I was confused as hell as well but after a few videos and research on my own, I finally understood.
Sometimes we just don’t need to question ourselves too much, ultrasound can determine the age of our fetus.
Take care!