Conjoined twins are identical twins whose bodies are connected together. Conjoined twins form during development when the egg is fertilized and begins to separate to form twins but does not do so completely. This happens in 1 in 50,000 to 200,000 births, and the outcomes for conjoined twins vary depending on how their bodies are joined and which organs they share. In some cases, conjoined twins may be stillborn. In other cases, the twins may be surgically separated. Separation surgery can be difficult and have a number of risks, though some conjoined twins have been successfuly separated and have gone on to live normal lives. Some conjoined twins have survived while remaining connected, because separation surgery presented too many risks.