Trying to stay positive . . .

So I went in today for my first doctors appointment to confirm the pregnancy of my second. I should be 7 weeks and 6 days based on the first day of my last period. The doctor says that he only measures me at 5 weeks and 6 days. I only got my pp period back in October and my last one was December before getting my BFP. The doctor mentioned that since I wasn’t off birth-control long enough, that could play a role in the difference in weeks but he also mentioned that the egg could have started to grow until week five and has now stopped. Any similar situations?