Abnormalities of the membranes include amniotic sheets and amniotic bands. Overall they affect around 0.8% of pregnancies. Sometimes these can cause problems for the baby, but usually the outcome is completely normal if everything else looks fine at the time of the scan.
Amniotic sheets are the most common, affecting about 1 in 200 pregnancies. They are thought to be caused by scarring inside the womb or to use the medical term, 'senechiae', from previous operations, such as a dilatation and curettage (D&C). As the baby's membranes develop, they surround these sheets of scar tissue. Often when another scan is done later in the pregnancy, they are no longer seen, presumably following rupture or compression by the growing baby. Amniotic sheets have in some studies been found to be associated with an increased risk of early labour. There is nothing you can really do about this apart from being alert to the possibility and reporting any pains or contractions early if they do start.
Amniotic bands are even less common, affecting 1 in 1200 (0.08%) of all pregnancies. They are strands of membrane stretching from the outer membrane surface into the amniotic cavity where the baby is floating in liquor. The membranes surrounding the baby is made up of 2 parts, an inner amnion and outer chorion. Amniotic bands are thought to originate when a tear occurs in the inner membrane without injury to the chorion. The ruptured amnion remains as a plaque or fragments into bands which stretch across the cavity. Amniotic bands are thought to happen either spontaneously or sometimes following some form of trauma to the abdomen. There is a suggested relationship between amniotic bands, and early chorion villous sampling (CVS). This is why CVS is not done until after 10 weeks of pregnancy.
As the baby develops, its extremities may become entangled in the amniotic bands resulting in immobilisation, trapping, constriction or even amputation. On ultrasound the bands appear as a thin, floating lines, which may be seen attached to or around the baby. About 70% of amniotic bands disappear on follow-up ultrasound, presumably due to rupture or compression.
If there is no evidence of any abnormality, other than the amniotic band, at the time of the scan then there appears to be little risk to the baby. Other reassuring factors are normal fetal movement and the band not being attached to the baby. In this case they have been called 'innocent amniotic bands'.
It is extremely unlikely that amniotic bands will affect a future pregnancy.