6 week scan after FET

Hi- this is my first time to post and I was wondering if you can share some insights with regards to my situation and the scan attached.

I had a 6 week scan a few days ago and the doctor didn't say anything except that the sac is too small. He said it should have been at 10mm vs 5.6mm. not sure if there's a standard size at 6 weeks?

There were no other observations except that he wanted me to come back next week.

It's been a tough journey so far, I had FET last 1/31 from the only normal pgd-tested 5/6 day blast we got. My bhcg are as follows:

2/8: 26

2/11: 63

2/13: 159

Everytime I go see the doctor, he always says that my results are lower than expected and warned me of a potential abnormal pregnancy.

Appreciate all the insights and support!