Need some help/ hope if anyone had something similar happen to them. Measuring behind

My lmp was 12/27 which would make me 6 weeks 1 day pregnant. I have been going to my doc for the last 3 weeks bc I had a mc in August and he wanted to keep on top of everything. I went for my first sono at 4 week and he did an internal and it showed thickening of my uterus. Went last week at 5 weeks 1 day showed a very tiny sac. Went yesterday at 6 weeks and the sac grew again but still no baby yet. My doctor is giving it the benefit of the doubt that I ovulated late and really am a week behind than what we thought. My progesterone levels at week 4&5 were good and hcg levels more than doubled. He didn’t have me retest them last week but as precaution I had more done today. Someone please tell me they had a positive outcome of out of a situation like this. Trying to keep positive 🙏🏻🤞🏻