Gestational sac too small?

Hello!

I had an early ultrasound today due to me being afraid of an ectopic. I am 20 DPO and 4weeks 5 days. I got my first vvvf at 8 DPO that didnt show up on camera and a vvf at 9 DPO that showerd up on camera.

Got my first DARK line today at 20 DPO that was similar to control line.

The gestational sac is in my uterus!!! It is not ectopic ❤️ However, he measured it as 4+1 weeks. Since i know when I ovulated and did not implant very late since I tested positive so early I am sure it should be a bit bigger?

It is my first pregnancy so I dont know these things too well, but should I be cautious? Or is the margin of error large at this stage?

I know its too early to see a yolk sac, but he said he could see a vague structure in there.

I did not get any explanation or answers so can anyone with experience help me 😅

Reddit wont let me scroll up to edit so sorry for spelling mistakes!