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 normal if everything else looks fine at the time of the scan.

Amniotic Sheets

Amniotic sheets are the most common with an incidence of 0.6%. They are thought to be caused by scarring inside the womb or 'senechiae' from previous operations, such as D&C. As the membranes develop, they surround these sheets of scar tissue. Often they aren't seen until later on in pregnancy, presumably following rupture or compression by the growing baby. Amniotic sheets have been found to be associated with an increased risk of early labour.

Amniotic Bands

Amniotic bands are even less common, affecting 1 in 1200 (0.08%) of all pregnancies. They are fibrous strands of membrane stretching from the outer membrane surface into the amniotic cavity. They are thought to originate when the inner membranes (amnion) rupture without injury to the outer membrane (chorion). The ruptured amnion remains as a plaque or fragments into bands which stretch across the chorionic cavity. Amniotic bands are thought to happen spontaneously or in association with trauma to the abdomen. There has been the suggestion of a relationship between amniotic bands, limb shortening and early chorion villous sampling (CVS).

Effects of Amniotic Bands

As the baby develops, its extremeties may become entangled resulting in immobilisation, trapping, constriction or even amputation. It really isn't known if the reports of limb reductions associated with early CVS are due to amniotic bands. On ultrasound the bands appear as thin, mobile 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'.

One study has looked at 25 cases of innocent amniotic bands and found a 4-fold increased risk of preterm delivery (before 37 weeks) and a 9-fold increased risk of a low birth weight infant. The numbers in this study were small and this is something that needs a larger study to confirm or refute.

It is extremely unlikely that amniotic bands will affect a future pregnancy.

References

Wehbeh H et al. The relationship between ultrasonographic diagnosis of innocent amniotic band development and pregnancy outcomes. Obstet Gynecol 1993;81:565-8

Mahonny BS et al. The amniotic band syndrome: Antenatal sonographic diagnosis and potential pitfalls. Am J Obstet Gynecol 1985;152:63-8

Randel S et al. Amniotic sheets. Radiology 1988;166:633-6

Brown D et al. Intrauterine shelves in pregnancy: Sonographic observations. AJR 1989;153:821-4