Correct, its sirenomelia and also called mermaid syndrome as answered by Sulav. I would like to put some facts about sirenomelia:
Fused legs giving appearance of mermaid
Incidence: 1 in 70,000 births
Rarely live births occur
While most normal fetuses have two umbilical arteries and one umbilical vein, the sirenomelia fetus just has one. Most often, if a sirenomelia baby does have two arteries (which is extremely rare) it is most often blocked. With one umbilical artery (vitelline artery) providing blood to the fetus, it is obvious that it grows abnormally large and fails to fuse when it is supposed to. This artery thus steals the blood and nutrition from the lower body of the fetus and diverts it back up the placenta. Due to this malnutrition, the lower limbs of the fetus fail to develop and separate into two limbs.