Conjoined twins

Conjoined twins, popularly referred to as Siamese twins, are twins joined in utero. It is a very rare phenomenon, estimated to occur in anywhere between one in 50,000 births... Wikipedia

• Symptoms:  Bodies fused
• Usual onset:  Beginning of pregnancy
• Duration:  Lifelong
• Causes:  Incomplete fission
• Treatment:  Surgery, symptomatic care
• Data source:  DuckDuckGo