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Sirenomelia is a lethal birth defect of the lower body characterized by apparent fusion of the legs into a single lower limb. Other birth defects are always associated with sirenomelia, most commonly abnormalities of the kidneys, large intestines, and genitalia.


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.

Genetic profile

All cases of sirenomelia have occurred in families as isolated cases, and there is no known genetic cause. It is possible that sirenomelia is an autosomal dominant condition and because it is lethal, all cases represent a new mutation. Alternatively, it might be a multifactorial trait where multiple genes and environmental factors come together to cause this pattern of malformations. The fact that all cases have been isolated does not support this possibility. Sirenomelia is more common in twin pregnancies. This may give evidence to an environmental cause.


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.

Signs and symptoms

Abnormalities associated with sirenomelia 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.

Treatment and management

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.


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.



De Silva, M.V., and W.D. Lakshman. "Sirenomelia Sequence (Mermaid Syndrome)." Ceylon Medical Journal 44 (March 1999): 34-5.

Stevenson, Roger E., et al. "Vascular Steal: The Pathogenic Mechanism Producing Sirenomelia and Associated Defects of the Viscera and Soft Tissues." Pediatrics 78 (September 1986): 451-457.

Stocker, J.T., and S.A. Heifetz. "Sirenomelia. A Morphological Study of 33 Cases and Review of the Literature." Pespectives in Pediatric Pathology 10 (1987): 7-50.

Valenzano, M., et al. "Sirenomelia: Pathological Features, Antenatal Ultrasonographic Clues, and a Review of the Current Embryogenic Theories." Human Reproductive Update 5 (January-February 1999): 82-6.

Randall Stuart Colby, MD