This is a video clip of a fetus at 36 weeks of gestation.

These are the antenatal scans of the same.

The clear finding is this floppy membrane floating in the amniotic fluid. Note that it is in intimate contact with the fetal limbs but it does not appear to constrict or restrict them. The fetus appears like a kid playing "ghost" and draped in a large sheet. And of course a critical finding is the absence of a twin, the usual cause for a "membrane".

This is typical of amnion rupture. The chorion is intact but the amnion is torn. When it occurs early in the pregnancy, it can result in the amnion-rupture sequence. Later the amnion can no longer "stick†to the fetus. It then presents as a veil floating around the fetus. This is similar to the "folding membrane sign†of the early twin-to twin transfusion, but of course without a co-twin.

Reference:

Jeanty P, Laucirica R, Luna SK. "Extra-amniotic pregnancy. A trip to the extraembryonic coelom." J Ultrasound Med. 1990 Dec;9(12):733-6.