Description, Causes and Risk Factors:

Sirenomelia is a birth defect that affects approximately 1 out of every 65,000 live births. The condition is also known as Mermaid syndrome, because the legs are fused together and the child will look like a mermaid (half woman and half fish). Mothers that have diabetes have an increased risk of carrying a baby with sirenomelia. Because the kidney and bladder will not be able to develop or function properly, babies that are born with this condition will generally die within a few days of birth.

However, there have been a few people that have survived because their kidney and bladder were not affected. In the case that a child's bladder and kidneys are not affected, surgery will be performed to separate the fused legs. A prenatal ultrasound examination can generally detect the condition before the child is born.

Sirenomelia is rare, estimated to occur once in every 60,000 births. While the exact incidence in different populations is not known, sirenomelia has been reported in a variety of ethnic groups around the world. It is known to be more common in twin pregnancies and in babies born to mothers with diabetes mellitus.

The exact cause of this condition is unknown. Genetics is believed to be one of the major causes, but there have been no known exact condition that lead to this disease. The mother suffering from a severe or major illness during pregnancy, suffering from diabetes mellitus, etc. believed to be some of the possible mermaid syndrome causes. Lack of proper prenatal care is also believed to be one of the sirenomelia causes. As this condition is found in people belonging to different races and ethnic groups, one can infer that it is not limited to any specific locality and can be found all over the world. Scientists are yet to find other specific causes of sirenomelia.

This pattern of birth defects is associated with abnormal umbilical cord blood vessels. The normal fetus develops two umbilical arteries, which pump blood from the fetus to the placenta, and one umbilical vein, which returns blood from the placenta to the fetus. The umbilical arteries branch off the iliac arteries in the pelvis. The iliac arteries supply the legs and pelvic organs such as the genitalia. Most babies with sirenomelia have only one umbilical artery and one vein. Rarely a baby with sirenomelia can have the typical two arteries and one vein with occlusion (blockage) of one artery.

In sirenomelia, the one functional artery is larger than normal and branches from the aorta high in the abdomen. Below this umbilical artery, the aorta becomes abnormally narrow. This type of single umbilical artery is known as a vitelline artery because it is thought to arise from the primitive vitelline arteries early in the life of the embryo. The vitelline arteries normally fuse a few weeks after conception to form the arteries that supply the gastrointestinal system and genitourinary system (superior mesenteric, inferior mesenteric, and celiac arteries). If the normal umbilical arteries do not form correctly as branches from the iliac arteries, then a vitelline artery might persist.

The vitelline umbilical artery steals blood and nutrition from the lower body and diverts it to the placenta. This results in a small aorta and variable absence of the arteries that supply the kidneys, large intestine, and genitalia (renal, inferior mesenteric, and celiac arteries). Because of the loss of nutrition and blood flow, the lower limbs fail to form as separate limbs, the kidneys do not form or are malformed, the large intestine ends blindly in the abdominal cavity, the anus is imperforate, and the internal and external genitalia are absent or malformed.

The typical malformation of the lower limbs seen in babies with sirenomelia consists of apparent fusion of the legs. There is a spectrum of severity with severe cases having one lower limb that tapers to a point with the absence of foot structures. In these severe cases there are only two bones present in the entire limb (a femur and presumably a tibia). On the mild end of the spectrum are babies with fusion of the skin of the lower limbs only. In these infants the feet may be fully formed with fusion at the ankles. All bones are fully formed and separate. Normally there are three bones in each leg — the femur in the upper leg (thigh) and the tibia and fibula in the lower leg (calf).

Other birth abnormalities of the upper body involving the heart, lungs, spine, brain, and arms can also be seen in this syndrome, however, not in every affected individual. It is unknown at this time why a single umbilical artery could cause these changes.

Single umbilical artery occurs in about 1% of all live-born infants. In most of these infants the one umbilical artery is normally formed and not of vitelline origin. In these cases, the risk of other birth defects is low (about 8%). All infants born with a vitelline umbilical artery will have other malformations, the most common being sirenomelia.

It has recently been shown that sirenomelia and its associated defects are produced by an alteration in early vascular development. Dissection of the abdominal vaculature in 11 cases of sirenomelia has demonstrated the presence of a single large artery arising from the aorta just below the diaphragm. The steal vessel derives from the vitelline artery complex, which is an early embryonic vascular network that supplies the yolk sac. This artery assumes the function of the umbilical arteries and diverts nutrients from the caudal end of the embryo distal to its site of origin. The abdominal aorta distal to the origin of the vessle is always subordinate and has no branches before it bifurcates into the iliac arteries. Consequently tissue dependent on nutrients supplies by this hypoplastic vascular system may be absent, arrested at an incomplete stage of development or abberantly formed. Thus, as with other disruptive vascular defects, no two cases of sirenomelia are the same.

It is possible to demonstrate sirenomelia by ultrasound as early as 20 weeks of pregnancy. As it is lethal an attempt should be made to identify this condition in cases of IUGR (intrauterine growth restriction) and/or oligohydramnios at this stage.

Because of the birth defects involving the gastrointestinal tract and kidneys, sirenomelia is almost always fatal. About 50% of babies are stillborn (the baby has died before delivery) and 50% are live-born with survival lasting a few minutes to a few days. There have been at least two reported cases of sirenomelia that have survived beyond the first month of life. These infants had normal functioning kidneys during their development.


Symptoms may include:

    Absence of the kidneys or malformed nonfunctioning kidneys.

  • Blind ending colon and imperforate anus.

  • Small, absent, fused, or poorly formed pelvic bones.

  • Small, absent, or poorly formed internal and external genitalia.

  • Fusion of the lower limbs along the inner leg, from skin only to complete fusion with the appearance of only one leg.

  • Death from underdeveloped and immature lungs caused by oligohydramnios.

  • Birth defects in the upper body sometimes occur and include abnormalities in the heart, lungs, arms, spine, and brain.


The diagnosis is obvious at birth on examination of a baby, but prenatal diagnosis often occurs in the second trimester (weeks 13 through 26 of a pregnancy) by an ultrasound.

The antenatal diagnosis of sirenomelia by X-ray and ultrasonography has been reported. Sometimes because of oligohydramnios the diagnosis of sirenomelia is missed out. As it is possible to demonstrate sirenomelia as early as 20 weeks of pregnancy and it being a lethal anomaly, an early diagnosis could be useful in terminating the pregnancy.


Babies born alive with functioning kidneys may survive with appropriate surgical management. Operations to reconstruct the urinary and gastrointestinal outlet tracts are almost always needed. Other procedures and treatments depend of the extent of other birth defects. It appears that if a baby does survive, he or she will not have any mental delays.

NOTE: The above information is for processing purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.


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