Small gestational sac


I am currently 13 weeks and 12 weeks with di/di twins :)

First pregnancy.

We are monitoring the small twin as it has a small gestational sac but other wise seems to be totally fine.

Getting an amniocentesis done at 16 weeks.

Has anyone else experience such a small sac with any of their babies?

For reference here is the other twin below with a spacious sac :)