Sirenomelia

Sirenomelia, sometimes called mermaid syndrome, is a rare congenital deformity in which a developing fetus's legs are fused together, producing the appearance of a single mermaid-like tail. Clinicians classify cases by the severity of the underlying skeletal fusion, ranging from a milder form in which all the leg bones are present and only the soft tissue is fused, to the most severe form in which only a single fused femur remains. The condition is often accompanied by kidney agenesis, urinary retention, and other malformations, and it has been linked to caudal regression syndrome and the group of birth defects known as the VACTERL association. The leading explanation for its cause is an insufficient blood supply reaching the developing lower limbs early in pregnancy, though researchers have also proposed embryonic injury occurring around weeks 28 to 32 of development, exposure to substances that disrupt fetal development, and defects arising during the twinning process; some studies link maternal diabetes to the condition, though the connection is disputed. Sirenomelia occurs in roughly one in 68,741 to one in 97,807 live births and is markedly more common in identical twins, and while surgery can sometimes separate the fused limbs, the condition is usually fatal, with most affected infants dying shortly after birth.