There are two theories of what causes conjoined twins:
- Fission: An early embryo, comprising a small sphere of identical cells, splits into two spheres but doesn’t separate completely. The two spheres each develop independently into the conjoined twins.
- Fusion: An identical twin pregnancy contains separate early twin embryo “spheres” that merge together and join at a random point of connection.
Both of these theories help explain how conjoined twins might form during the first 12-14 days of fertilization. But scientists still don’t know why this happens.