*3.3.3. Sirenomelia (meromelia)*

Sirenomelia, named after the Sirens (half-bird women in Greek mythology, often confused with mermaids), is an extremely rare form of malformation of the extremities, characterized by fused lower limbs hence resembling "merpeople" (**Figure 4D** and **E**) [29]. With only a single umbilical artery and vein, it is difficult for both limbs to develop, resulting in the formation of fused lower limbs [30]. It can be divided into categories, based on the degree of development of the lower limb: sympus apus, sympus monopus, and sympus dipus, referring to absence of feet, short feet, and a pair of feet, respectively. Anomalies of the kidneys, large intestine, and external genitalia are commonly observed as accompanying complications.

This lethal congenital anomaly begins to show up at CS 13, with an occurrence rate of approximately 1.5–4.2 in 100,000 births, with more than half of them born dead. All such cases lead to death within 5 h after birth [29]. Although chromosomal aneuploidy is not associated, maternal diabetes mellitus and monozygotic twins are considered important factors for increasing the risk of sirenomelia [31].
