It is likely that your scan picked up a harmless amniotic sheet, rather than an amniotic band. Structures within the amniotic fluid are often seen during routine scanning, and are divided into two categories:
amniotic sheets (folds), which are common
amniotic bands (threads), which are rare
There are other important differences between them.
Amniotic sheets don’t harm you or your baby. They are found in about one in 200 pregnancies, and are caused by the membranes folding over and attaching to scar tissue in the uterus (womb). These may form if you have previously had an operation such as a dilatation and curettage (D&C), where tissues are taken from your uterus wall. Amniotic sheets may also form if you have had an infection.
On a scan, amniotic sheets look like thick walls or pillars in the amniotic fluid. Because the sheet is separate from your baby, it won’t affect his development. Though in rare cases, it may divide a large area of the uterus and restrict your baby's movements. This may cause your baby to lie in a breech position, or mean that you go into early labour.
If the sonographer does find an amniotic sheet, she will probably ask you to return for a growth scan to check your baby’s movements and position. The scan will also confirm whether or not it is an amniotic band.
Amniotic bands, also called threads, can be more serious, but are extremely rare. The way they form has to do with the two layers of membrane in the amniotic sac. These are called the inner amnion and the outer chorion, and they are separate in early pregnancy.
The layers should fuse by the time you are 16 weeks pregnant, but if the amnion tears before it fuses with the chorion, it can form strands that wrap around your baby. This may cause serious abnormalities, because it disrupts your baby’s blood supply (amniotic band syndrome). On a scan, very thin strands can be seen floating in the amniotic fluid, and may be attached to parts of your baby.
The early rupture of the amnion is usually a chance happening, but it can also happen if you have surgery on your baby while he is in your uterus (womb), or if you have an amniocentesis.
If your sonographer saw no abnormalities other than the amniotic sheet at the time of your scan, it’s usually nothing to worry about.
Knowing that your baby’s movements are normal, and that the membranes aren’t attached to your baby, should reassure you. As your baby grows, the sheet will most likely be compressed against the wall of your uterus, and will no longer be seen