Conjoined twins cannot be different sexes, but 70% are female
Because conjoined twins start out as one egg that does not fully separate into two individuals, the gender of each sibling is the same; you cannot have conjoined twins of differing genders. Moreover, 70% of conjoined births are female, occurring in a ratio of three to one. However, medical professionals are unsure why this gender predominates. Again, due to the rarity of this genetic malformation, scientists are experiencing a dearth of information to better understand the condition.
Ambroise Pare and Geoffrey St. Hillaire were among the first teratologists (scientists who study physiological abnormalities) to identify and name the types of conjoined twins. As such, there are 10 recognized versions of conjoined twinning, all of which are associated with where the siblings are conjoined. For example, Abby and Brittany Hensel are considered “dicephalic” twins, while Millie and Christine McKoy were pygopagic. Throughout history, the cases of conjoined twins have more often than not been female, although the most famous pair, Eng and Chang Bunker, were male.