A rare type of "semi-identical" twinning has been identified by scientists reporting in the Journal of Human Genetics. A set of three-year-old twins are believed to have formed when two sperm simultaneously fertilized a single egg, and then the egg split. The situation is distinguished from polar body twinning a theory that describes a type of twinning that occurs when an egg that splits before fertilization and is fertilized by two different sperm.

According to the report, the twins were conceived without assistance and are healthy, however one child is a hermaphrodite with both ovarian and testicular tissue and is being raised as a female. The other twin is male.

• Read more at Nature.com
• More Unique & Unusual Types of Twins