Amniotic Band Syndrome: What a Scan Finding Means

What is amniotic band syndrome?

Amniotic band syndrome (ABS), also called amniotic band sequence, is a group of differences in a baby's body that happen when fibrous strands in the womb wrap around part of the baby and restrict its growth or blood supply. A review in the Journal of Prenatal Medicine describes it as "a wide range of congenital anomalies, most typically limb and digital amputations and constriction rings, that occur in association with fibrous bands".

It ranges from a constriction ring on a finger to changes affecting several parts of the body. Because the outcome depends so much on what the band has touched, a diagnosis on a scan is the start of a conversation with a specialist, not a verdict.

What causes amniotic band syndrome?

Nobody knows for certain. A large European study of 866 cases says plainly that the cause of ABS "is unknown". The Journal of Prenatal Medicine review says the causes are mostly still unknown, suggests a mechanism involving damage to blood vessels, and lists factors that have been studied, including smoking, drug use, raised blood sugar in pregnancy and puncture of the amniotic sac after amniocentesis.

A few links are better established:

• Twins. The European registry study (EUROCAT, 30 registries, 1980–2019) confirmed twinning as a risk factor. Our page on types of twins and chorionicity explains how twin pregnancies are monitored.
• After some womb procedures. In a Dutch series of 672 twin pregnancies treated with laser surgery for twin-to-twin transfusion syndrome, 2.2% had amniotic bands at birth.
• Age and first pregnancies. An Australian study found ABS more often in mothers under 25 and in first births.

Even so, most people with these factors never have it.

How common is it?

It is rare. EUROCAT recorded an average of 0.53 cases per 10,000 births across Europe over 40 years, counting live births, stillbirths and terminations for fetal anomaly. Rates were higher in the United Kingdom than the European average. A population study in Western Australia and South Australia found 2.03 per 10,000 births.

Studies find and define cases differently, which is one reason estimates online disagree so widely. In the Australian study, 15 of the 40 cases were found only by the researchers' own search, not by the birth defects registries.

Amniotic band syndrome on an ultrasound

ABS is usually suspected from what the band has done rather than from the band itself. Findings that raise it include a ring-shaped narrowing around a limb, swelling below that ring, missing fingers or toes, or visible strands of membrane. A Dutch study describes the condition as "constriction of the limbs or umbilical cord by fibrous strands, leading to edema, amputation, and/or fetal demise".

The band itself is not always visible. In a US series of 28 confirmed cases, membrane disruption was recorded in 11, and the authors concluded that "membrane disruption is not always present".

If ABS is suspected, it is reasonable to ask to be seen by a fetal medicine specialist for a detailed scan. You can find what the routine mid-pregnancy scan looks at on our 20-week scan page.

A band, or an amniotic sheet?

Not every line across the womb on a scan is an amniotic band. An amniotic sheet is a fold of membrane over scar tissue in the womb. A Singapore study of 30,476 scans found sheets in 0.14% of pregnancies, and concluded that incomplete sheets, the usual kind, "are benign". The study did find that the two complete sheets, which had no free edge, were both linked to the baby dying in the womb, so ask your sonographer exactly which has been seen.

Even a true band on a scan does not always cause harm. In a US study of 25 pregnancies with a band seen on ultrasound and normal anatomy, no babies had anomalies, although preterm birth and low birth weight were more common.

What it can affect: hands, fingers, face and cord

The limbs are affected most often. In the US series, 20 of 28 cases involved the arms or legs, 7 the umbilical cord, 5 the abdomen, and 1 each the head and chest. EUROCAT found limb differences and neural tube defects were common in ABS.

The face and head can be involved. In the Australian study of 40 babies, 24 had limb differences only, while 12 had complex differences of the face and skull with other features.

Cord involvement is the most serious. A band around the umbilical cord can cut off the baby's blood supply. In the US series, cord involvement was found in 67% of babies who did not survive against 19% of those who did. Overall survival there, excluding three terminations, was 74%.

Can it be treated before birth?

Sometimes. Surgeons can pass a fine telescope into the womb (fetoscopy) and cut the band, but it has been reported in only a few dozen cases, so the evidence is limited to small series.

• A 2024 systematic review in Prenatal Diagnosis found 37 reported cases across 17 studies. Surgery was done at a median of 22 weeks and preserved the affected limb and its function in 75.7%.
• The review says the procedure can prevent fetal death when the cord is involved, but "is not free of perinatal complications", and that criteria for choosing who benefits are still unclear.
• In the US series, 9 of 28 pregnancies were judged suitable for surgery; 5 had it, and 4 of those babies survived.

Whether surgery is offered depends on what is affected, how severely, and where you are. Ask your specialist what the options are in your region, including referral elsewhere, and what follow-up your baby will have after birth. Our page on findings on a scan covers how unexpected results are discussed.

Getting support

A finding like this, often at a scan you expected to be routine, is hard to hear. You can ask for time, for the images to be explained, and for a second appointment once you have had time to think. If further tests such as amniocentesis are suggested, ask what each would tell you and what it would change. Our guide to screening in the UK sets out what is routinely offered.

Frequently asked questions

What causes amniotic band syndrome?

The cause is unknown. The large European EUROCAT study of 866 cases says so directly. Researchers think damage to the blood supply of the affected part is involved. Twin pregnancy is a confirmed risk factor, and bands are occasionally seen after procedures inside the womb, but in most cases there is no explanation.

Can amniotic band syndrome be seen on an ultrasound?

Often, but indirectly. It is usually suspected from a narrowing ring, swelling beyond it, or missing fingers or toes, rather than from the band. In one US series, membrane disruption was recorded in only 11 of 28 confirmed cases. A thin line across the womb may instead be an amniotic sheet, which is usually harmless.

Can amniotic band syndrome affect the face?

It can, though limbs are affected far more often. In an Australian population study, 12 of 40 babies had complex differences involving the face and skull along with other features, while 24 had limb differences only. A specialist scan can show which areas are involved.

Is amniotic band syndrome treatable?

In selected cases, a band can be cut before birth by fetoscopic surgery. A 2024 review of 37 such operations found the affected limb and its function preserved in 75.7%, but the evidence comes from small series and the procedure carries risks of its own. Other care is planned after birth, depending on what is affected.