What are conjoined twins?

Conjoined twins are whose bodies are joined (skin & internal organs are fused together), which is a rare phenomenon. This rare from of identical twins occurs one out of every 75,000 to 100,000 births or 1 in 200 deliveries of identical twins, that end up conjoined.

Conjoined twins originate from a single efertilized egg, so they are always identical and same-gender twins. The developing embryo starts to split into identical twins within the first two weeks after conception, but then stops before completion, leaving a partially seperated egg which continues to mature into a conjoined fetus.

The birth of conjoined twins is chaotic. Approximately 40%-60% of these births are delivered stillborn with 35% surviving just one day. The overall survival rate of conjoined twins is somewhere between 5%-25% and historical records over the past 500 years detail about 600 surviving sets of conjoined twins, with more than 70% of those pairs being female.

While there are dozens of types of conjoined twins, doctors divide the types into more common variations. All of these types can be more broadly categorized as displaying either equal and symmetrical forms, or unequal and possible asymmetrical forms.