Scientists identified a boy and girl, four, from Brisbane, Australia, who are in between fraternal and identical twins. The pair of twins, from the same egg, shared the same placenta and grew in the same amniotic sac in their mother’s womb – which is only ever possible with identical twins. However, the twins are male and female, meaning they cannot be truly identical but instead are “semi-identical”. Professor Nicholas Fisk, who cared for the mother and her twins at the Royal Brisbane and Women’s Hospital in 2014, made the remarkable discovery.
The woman, who was 28 when she gave birth, conceived naturally.
The twins only share a portion of their father’s DNA and are identical on their mother’s side.
According to the New England Journal of Medicine who reported the case: “The mother’s ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins.
“However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins.”
Semi-identical twins are formed when a male and female sperm fuse with the mother’s egg at the same time – but these embryos rarely survive.
Usually, either an X or Y sperm will successfully fuse with an egg – but this time, both sperms fertilised with the egg simultaneously.
Identical twins occur when one egg is fertilised by an X or Y sperm and divides, forming a pair of twins sharing the same gender and other similar traits – including hair colour, eye colour and facial features.
Non-identical twins are formed when a mother releases two eggs, with two separate sperm cells – carrying either an X-chromosome or Y-chromosome – fertilising each egg.
Non-identical twins, or dizygotic twins, can share completely different genetic features.
This is the second case in the world recorded. The first pair of semi-identical twins were born in the US in 2007.
Infertility treatment (IVF) can increase a mother’s likelihood to conceive twins, triplets and even quadruplets while older mothers are also more prone to having twins.
Professor Fisk added: “We know this is an exceptional case of semi-identical twins.
“While doctors may keep this in mind in apparently identical twins, its rarity means there is no case for routine genetic testing.”