• Disease Overview
  • Synonyms
  • Signs & Symptoms
  • Causes
  • Affected Populations
  • Disorders with Similar Symptoms
  • Diagnosis
  • Standard Therapies
  • Clinical Trials and Studies
  • References
  • Programs & Resources
  • Complete Report



Last updated: September 19, 2016
Years published: 1990, 2002, 2012, 2016


NORD gratefully acknowledges Susan E. Crawford, DO, Director, Metabolic Core Facility, NorthShore University Research Institute, University of Chicago Pritzker School of Medicine, for assistance in the preparation of this report.

Disease Overview

Sirenomelia, which is also known as mermaid syndrome, is an extremely rare congenital developmental disorder characterized by anomalies of the lower spine and the lower limbs. Affected infants are born with partial or complete fusion of the legs. Additional malformations may also occur including genitourinary abnormalities, gastrointestinal abnormalities, anomalies of the lumbarsacral spine and pelvis and absence or underdevelopment (agenesis) of one or both kidneys. Affected infants may have one foot, no feet or both feet, which may be rotated externally. The tailbone is usually absent and the sacrum is partially or completely absent as well. Additional conditions may occur with sirenomelia including imperforate anus, spina bifida, and heart (cardiac) malformations. Sirenomelia is often fatal during the newborn period. The exact cause of sirenomelia is unknown, most cases occur randomly for no apparent reason (sporadically).

Some sources in the medical literature classify sirenomelia as the most severe form of caudal regression syndrome, a complex developmental disorder. However, recently many researchers have indicated that sirenomelia is a similar, but distinct, disorder. NORD has a separate report on caudal regression syndrome.

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  • mermaid syndrome
  • sirenomelia sequence
  • sirenomelia syndrome
  • sirenomelus
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Signs & Symptoms

There are a wide range of physical malformations that can potentially occur with sirenomelia and the specific findings can vary greatly from one individual to another. Sirenomelia is associated with severe life-threatening complications and is often fatal in the first years of life. However, survival beyond infancy into later childhood or young adulthood has been reported in a handful of cases.

The characteristic finding of sirenomelia is partial or complete fusion of the lower legs. The degree of severity is highly variable. Affected infants may have only one femur (the long bone of the thigh) or may have two femurs within one shaft of the skin. Affected infants may have one foot, no feet or both feet, which may be rotated so that the back of the foot is facing forward.

Affected infants may also have a variety of urogenital abnormalities including absence of one or both kidneys (renal agenesis), cystic malformation of the kidneys, an absent bladder, narrowing of the urethra (urethral atresia). In addition, they may have an imperforate anus, a condition in which a thin covering blocking the anal opening or the passage that normally connects the anus and lowest part of the large intestine (rectum) fails to develop.

Infants with sirenomelia may also have abnormalities affecting the sacral and lumbar spine. In some patients, abnormal front-to-back curvature of the spine (lordosis) may occur. Affected individuals may also lack external genitalia. Absence of the spleen and/or the gallbladder has also been reported.

Defects affecting the abdominal wall may also occur such as protrusion of a portion of the intestines through a hole near the bellybutton (omphalocele). Some individuals with sirenomelia may have a meningomyelocele, a condition in which the membranes that cover the spine and, in some cases, the spinal cord itself protrude through a defect in the spinal column. Congenital heart defects and respiratory complications such as severe underdevelopment of the lungs (pulmonary hypoplasia) can also be associated with sirenomelia.

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The exact cause of sirenomelia is unknown. Researchers believe that both environmental and genetic factors may play a role in the development of the disorder. Most cases appear to occur randomly for no apparent reason (sporadically), which suggests environmental factors or a new mutation. Most likely, sirenomelia is multifactorial, which means that several different factors may play a causative role. In addition, different genetic factors may contribute to the disorder in different people (genetic heterogeneity).

The environmental factors that play a role in the development of sirenomelia are unknown. Some individuals may have a genetic predisposition to developing the disorder. A person who is genetically predisposed to a disorder carries a gene (or genes) for the disease, but it may not be expressed unless it is triggered or “activated” under certain circumstances, such as due to particular environmental factors. Researchers believe that environmental or genetic factors have a teratogenic effect on the developing fetus. A teratogen is any substance that can disrupt the development of an embryo or fetus.

In some individuals, sirenomelia is theorized to result from irregularities in early development of the blood circulating system (a disruptive vascular defect of the development of the vascular system) within the embryo. Some affected individuals have been found to have a single large artery arising from high in the abdominal cavity without the usual two arteries that normally branch out of the lower part of the aorta and carry blood to the rearward tail (caudal) end of the embryo. The single artery present (called a “steal” vessel since it essentially steals blood from the lower portion of the embryo) diverts the flow of blood which normally circulates from the aorta to the lower parts of the embryo and to the placenta. Thus the ‘steal’ vessel redirects the blood flow to the placenta without ever reaching the tail (caudal) end of the embryo. As a result of this rerouted blood flow, the steal vessel also diverts nutrients away from the blood-deprived portion of the embryo. Arteries in this caudal area are underdeveloped and tissues dependent upon them for nutrient supply fail to develop, are malformed, or arrest their growth in some incomplete stage. In individuals with sirenomelia, the lower limb bud of the embryo fails to divide into two legs. The underlying reason why these irregularities occur is unknown.

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Affected populations

Sirenomelia affects males more often than females by a ratio of 2.7-1. The exact incidence is unknown, but sirenomelia is estimated to occur in approximately 1 in 60,000 to 100,000 births. Sirenomelia occurs with greater frequency in one twin of identical (monozygotic) twins than it does in fraternal (dizygotic) twins or individuals.

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A diagnosis of sirenomelia can be made prenatally, most often during the second trimester, by fetal ultrasound. An ultrasound is an exam that uses high-frequency sound waves to produce an image of the developing fetus. A fetal ultrasound can detect some of the defects associated with sirenomelia.

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Standard Therapies


Treatment may require the coordinated efforts of a team of specialists. Pediatricians, surgeons, cardiologists, orthopedists, orthopedist surgeons, kidney specialists (nephrologists) and other health care professionals may need to systematically and comprehensively plan an affected child’s treatment.

Surgery has been successful in separating joined legs. In preparation for surgery, balloon-like tissue expanders are inserted under the skin. When they are filled with a salt solution over a period of time, the balloons expand making the skin stretch and grow. The excess skin is then used to cover the legs once they are separated. Sirenomelia is usually fatal in the newborn period despite treatment.

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Clinical Trials and Studies

Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov . All studies receiving U.S. Government funding, and some supported by private industry, are posted on this government web site.

For information about clinical trials being conducted at the NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:
Tollfree: (800) 411-1222
TTY: (866) 411-1010
Email: prpl@cc.nih.gov

For information about clinical trials sponsored by private sources, contact:

For information about clinical trials conducted in Europe, contact:

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Chen H, ed. Atlas of Genetic Diagnosis and Counseling. Humana Press. Totowa, NJ; 2006:905-906.

Jones KL, ed. Smith’s Recognizable Patterns of Human Malformation, 4th ed. WB Saunders Company. Philadelphia, PA; 1998:634.

Guven MA, Uzel M, Ceylaner S, et al. A prenatally diagnosed case of sirenomelia with polydactyly and vestigial tail. Gen Couns. 2008;19:419-424.

Das BB, Rajegowda BK, Bainbridge R, Giampietro PF. Caudal regression syndrome versus sirenomelia: a case report. J Peritnatol. 2002;22:169-170.

Van Keirbilck J, Cannie M, Robrechts C, et al. First trimester diagnosis of sirenomelia. Prenat Diagn. 2006;26:684-686.

Browne M, Fitchev P, Adley B, Crawford SE. Sirenomelia with an angiomatous lumbosacral myelocystocele in a full-term infant. J Perinatol. 2004;24:329-331.

Stanton MP, Penington EC, Hutson JM. A surviving infant with sirenomelia (Mermaid syndrome) associated with absent bladder. J Pediatr Surg. 2003;38:1266-1288.

Valenzano M, Paoletti R, Rosai A, et al. Sirenomelia. Pathological features, antenatal ultrasonographic clues, and a review of current embryogenic theories. Hum Reprod Update.1999;5:82-86.

Clarke LA, Stringer DA, Fraser GC, Yong SL. Long term survival of an infant with sirenomelia. Am J Med Genet.1993;45:292-296.

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Programs & Resources

RareCare® Assistance Programs

NORD strives to open new assistance programs as funding allows. If we don’t have a program for you now, please continue to check back with us.

Additional Assistance Programs

MedicAlert Assistance Program

NORD and MedicAlert Foundation have teamed up on a new program to provide protection to rare disease patients in emergency situations.

Learn more https://rarediseases.org/patient-assistance-programs/medicalert-assistance-program/

Rare Disease Educational Support Program

Ensuring that patients and caregivers are armed with the tools they need to live their best lives while managing their rare condition is a vital part of NORD’s mission.

Learn more https://rarediseases.org/patient-assistance-programs/rare-disease-educational-support/

Rare Caregiver Respite Program

This first-of-its-kind assistance program is designed for caregivers of a child or adult diagnosed with a rare disorder.

Learn more https://rarediseases.org/patient-assistance-programs/caregiver-respite/

Patient Organizations

National Organization for Rare Disorders