Hi Linh,
This is a good question.
Personally I think your example is a synechiae causing an amniotic sheet.
A synechiae is just an adhesion across the endometrial cavity and if they are multiple it is Ashermans syndrome.
When the gestational sac forms the adhesions cause this appearance as the amnion and the chorion must wrap around them essentially causing 4 layers of membrane hence the thick “band” appearance.
I think of these as exactly the same thing as an amniotic shelf.
Circumvallate placenta is different where the implantation site is too small for the placenta mass in simple terms and hence the membranes are puckered and protrude forward at the edge of the placenta.
Amniotic band syndrome is completely different again and this indeed causes limb deformaties. The limb issues occur due to the fetus coming in contact with the back surface of the amnion (the edge that is normally against the chorion) So this requires a perforation of the amnion and the floating membrane is really thin and whispy, not like your example.
So I think yours will just be a shelf caused by a synechiae and will be nothing to worry about.
Let me know what the tertiary referral folk think!
Steve.