First ultrasound appt

My husband and I have been trying for a year and a half to conceive. We found out a few weeks ago that we are expecting! By the date of my LMP my due date should be 2/3/16. I went to my first ultrasound appt today (6w2d) and my doctor said that he could not hear the heartbeat or see the baby yet and I have to return in a week to make sure the pregnancy is "viable". He saw the gest sac and a yolk sac. He thinks I'm actually only 5w5d pregnant. Has this ever happened to anyone else? It has me a little worried.