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Key Points

  • Incidence is 1 in 60,000 livebirths.

  • Results from localized injury or insult to caudal end of developing embryo between days 13 and 22.

  • Classified into 7 types according to the number of long bones present in the lower extremity.

  • Associated with monozygotic twinning.

  • First trimester sonographic findings include fused lower limb and increased nuchal translucency.

  • Second trimester sonographic findings include single umbilical artery, bilateral renal agenesis, oligohydramnios, and growth restriction.

  • Differential diagnosis includes bilateral renal agenesis and caudal regression syndrome.

  • Chromosomes are usually normal.

  • Prognosis is extremely poor for extrauterine survival.


Sirenomelia, also known as “mermaid syndrome,” has been noted since the Greco-Roman period. Initially described in the medical literature by Rocheus in 1542 (Murphy et al., 1992), the condition is characterized by a single lower extremity, with the concomitant presence of severe anomalies of the urogenital and gastrointestinal system. Although historical and mythological accounts portray sirens and mermaids as females, the majority of patients with sirenomelia are male (deJonge et al., 1984). In all likelihood, the confusion originated as a result of the fact that most patients with sirenomelia had no obvious external genitalia.


It is often stated that sirenomelia is characterized by apparent fusion of the lower limbs. This terminology is embryologically incorrect because fusion refers to two processes joining after breakdown of intervening epithelia. Merging is the more correct term, because it does not imply intervening breakdown of epithelium. Sirenomelia, therefore, is a syndrome of merging, malrotation, and dysgenesis of the lower extremities (Kapur et al., 1991).


In 1865, Förster classified sirenomelia into three groups according to the number of feet present (van Zalen-Sprock et al., 1995). In symelia apus, the most common of the three conditions, both legs are merged completely into a single lower extremity. Both feet are absent or rudimentary. On radiographic studies, only one femur is present; there are no fibulae and one or two tibulae. In symelia unipus, 1 foot is present and up to 10 toes also may be seen (see Figure 87-1). In this type of sirenomelia, two femora, two tibia, and two fibulae are present. In symelia dipus, two distinct feet are present, although generally they are malrotated, and often give the appearance of fins. Stocker and Heifetz (1987) further classified sirenomelia according to seven types. In type I, all thigh and leg bones are present. In type II, there is a single fibula. In type III, there is an absent fibula. In type IV, the femurs are partially fused and the fibulae are fused. In type V, the femurs are partially fused. In type VI, there is a single femur and a single tibia. In type VII, there is a single femur and an absent tibia.

Figure 87-1

A. Fetus with symelia apus, demonstrating a single lower extremity and a single ...

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