This case report demonstrates an early detection of sirenomelia at 12 weeks' gestation. Sirenomelia, also known as “mermaid syndrome,” has been noted since the Greco-Roman period. As with the mermaid mythology, sirenomelia is characterized by a single lower extremity with varying appearances of severity of the lower extremity anatomy. Mermaid syndrome is not considered a genetic defect but is associated with monozygotic twinning and maternal diabetes. Sirenomelia is a rare disorder that is difficult to diagnosis by sonography due to oligiohydramnios usually associated with the disorder. This case represents a late first-trimester diagnosis of sirenomelia without oligiohydramnios.
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