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!
Sometimes, when it is too early, the test cannot pick up the hcG levels. But actually you have already conceived. Its okay. Enjoy your pregnancy and congratulations!