Conjoined twins are twins whose tissues and organs are fused together.

Here are 5 things to know about conjoined twins, according to the University of Maryland Medical Center.

  1. Conjoined twins occur once every 200,000 live births.
  2. The survival rate of conjoined twins is between 5 and 20 percent.
  3. Approximately 70 percent of all conjoined twins female. Females are three times more likely than males to be born alive.
  4. Conjoined twins are generally identical so they're always the same sex.
  5. Conjoined twins form when a single egg produced by a woman doesn't fully separate after fertilization. The embryo starts to split into identical twins but never finishes the process. The partially separated eggs form a conjoined fetus.


According to the American Pediatric Surgical Association (APSA), there are five common forms of conjoined twinning. In order of frequency the types of conjoined twinning are thoracopagus (chest), omphalopagus (abdomen), pygopagus (rear pelvis), ischiopagus (front pelvis), craniopagus (head).