Mermaid syndrome, or sirenomelia, is a set of genetic abnormalities that cause a baby to be born with fused limbs, which may resemble a mermaid’s tail. The condition is very rare, with only 300 reports of this condition occurring in the world and is often fatal.

Mermaid syndrome is a medical condition that causes a baby’s legs to be fused together. Other names for mermaid syndrome include:

  • sirenomelia sequence
  • sirenomelia syndrome
  • sirenomelus

Mermaid syndrome is a very rare disorder affecting 0.8 to 1 baby per 100,000 live births. Male babies experience this condition three times more often than females.

Sirenomelia is a severe form of caudal regression syndrome, a term for disorders that involve problems forming the lower part of the spine. The most distinguishing characteristic of mermaid syndrome is legs that are either partly or completely fused together.

However, babies born with mermaid syndrome also tend to have other anomalies that affect their abilities to survive outside the womb. These include:

  • absent tailbone
  • anal atresia
  • foot abnormalities that include the absence of feet or feet rotated outward
  • gastrointestinal abnormalities
  • heart malformations
  • partly or completely missing sacrum
  • spina bifida
  • underdeveloped or absent kidneys
  • underdeveloped lungs

The severity by which mermaid syndrome affects a baby can vary. Once a doctor identifies that a baby has mermaid syndrome, they will often perform further testing to determine if other genetic abnormalities are present.

Doctors will typically diagnose mermaid syndrome during pregnancy. They may perform an ultrasound to view the fetus. At that time, a doctor may identify the abnormalities associated with mermaid syndrome, especially the fused lower limbs.

Journal articles have reported doctors diagnosing mermaid syndrome as early as 13 weeks of development.

Once a doctor identifies all the abnormalities that occur along with mermaid syndrome, they will speak to the parents about treatment options, if any are available.

Doctors don’t know exactly what causes mermaid syndrome and caudal regression syndrome, but they believe it’s likely a combination of environmental and genetic factors, according to the National Organization for Rare Disorders.

Doctors call many cases “sporadic,” meaning they occur for no known reason. These causes often lead to impaired blood flow in the embryo, specifically in the region where the lower limbs would typically grow.

Some potential environmental causes of mermaid syndrome and other caudal regression syndrome types include:

  • alcohol use
  • amino acid imbalances
  • lack of oxygen in the uterus
  • exposure to toxins, including those in cigarettes, lithium, and cocaine
  • exposure to retinoic acid

Researchers also associate differences in a specific gene called VANGL1 with increased risk for mermaid syndrome. However, doctors don’t know exactly why a mutation in this gene causes caudal regression syndrome.

Because doctors don’t know exactly what causes mermaid syndrome, there are no certain ways to prevent the condition. Doctors think maintaining steady blood sugar levels, especially in the first trimester, may help prevent the condition from occurring.

Doctors don’t know exactly what causes mermaid syndrome. They have identified one risk factor as maternal diabetes. An estimated 22 percent of fetuses with mermaid syndrome are born to someone with maternal diabetes. Unfortunately, doctors don’t know why diabetes increases the risk of having babies with mermaid syndrome.

Pregnant people over age 40 or younger than age 20 are also more likely to have babies with mermaid syndrome.

Treatment for a baby with mermaid syndrome usually involves seeing multiple medical specialists. The specialties depend on the syndrome’s effects. Orthopedic surgeons have performed procedures to separate fused limbs. However, even with treatment, mermaid syndrome is typically fatal past the newborn period.

Leg bones in those children with mermaid syndrome past this period are often fragile and prone to breakage. They may require multiple surgeries, both for their lower body and for any other genetic abnormalities, such as kidney disorders.

Sadly, because mermaid syndrome often causes other organ abnormalities, including those of the heart and lungs, mermaid syndrome is typically fatal in the newborn stage.

However, if mermaid syndrome mostly affects the legs and not other parts of the body (especially the kidneys), some babies may survive for a longer period.

Mermaid syndrome is a severe and often fatal congenital abnormality. Doctors emphasize getting regular prenatal care to enhance diagnosis and detection.

If you are concerned about this condition, your doctor can talk with you about medications and substances that can cause congenital abnormalities, including mermaid syndrome. Ideally, this can help to prevent congenital conditions whenever possible.