A set of Australian twins — a boy and a girl — have been identified as just the second in the world to be created from one egg and two sperm, and the first to be discovered during pregnancy.

Semi-identical twins are extraordinarily rare, according to New England Journal of Medicine report on the twins born in Queensland four years ago.

They are the first case to be seen in Australia; however, it has taken researchers several years to confirm they are in fact sesquizygotic.

The twins share 100% of their mother’s genes but only a proportion of their father’s DNA.