Extremely rare semi-identical twins identified in pregnancy

Australia's Queensland University of Technology has announced that a pair of young Brisbane twins, a boy and girl, have been identified as only the second set of semi-identical twins in the world, and the first to be identified by doctors during pregnancy.

The statement adds, now four year old, the boy and girl are identical on their mother's side sharing 100 percent of their mother's DNA, but are like siblings on their father's side, sharing only a portion of their father's DNA.

Professor Nicholas Fisk who led the fetal medicine team that cared for the mother and twins, is quoted as saying: "It's likely the mother's egg was fertilised simultaneously by two of the father's sperm before dividing."

The press release notes, the case, the first worldwide to identify some identical twins on genetic testing while in the womb, has been reported in The New England Journal of Medicine .

"The mother's ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins," Prof Fisk said.

"However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins."

The three types of twins

Identical twins occur when one egg is fertilised by a single sperm but divides and makes two babies.

Non-identical twins occur when two separate eggs are fertilised, each by a different sperm, and develop in the womb at the same time.

These twins can be the same or different sexes and are no more alike than any brother or sister, despite being born together.

However, in this case of semi-identical twins, the egg is thought to have been fertilised simultaneously by two sperm before it divided.

If one egg is fertilised by two sperm, it results in three sets of chromosomes, rather than two, this typically means the embryos do not survive to birth, which makes this case in Australia so rare.

The identity of the twins has yet to be revealed.