Australian doctors have identified a set of semi-identical twins -- the first set to be ever identified during gestation, and only the second known case of sesquizygotic twins in the world.

Semi-identical or sesquizygotic twins represent a third type of "twinning" apart from the identical (zygotic) and fraternal (dizygotic) twins. It's an extremely rare phenomena that results when two sperms fertilise the same egg. The Australian pair, born in 2014, was the first to have been observed in the womb.

The twins -- a boy and a girl -- were born at the Royal Brisbane and Women's Hospital, according to the report in The New England Journal of Medicine (NEJM). The five-year-old boy and girl are identical (monozygotic) on their mother's side, sharing 100 per cent of their mother's DNA, but are like siblings on their father's side, sharing only a proportion of their father's DNA.

"It is likely that the mother's egg was fertilised simultaneously by two of the father's sperm before splitting," said Professor Nicholas Fisk at the University of New South Wales (UNSW), who led the foetal medicine team at the hospital. According to Fisk, the mother's ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins. But an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins.

If one egg is fertilised by two sperms, it results in three sets of chromosomes, one from the mother and two from the father, which are typically incompatible with life and the embryos usually do not survive, explained Michael Gabbett, clinical geneticist at Queensland University of Technology (QUT).

But in this case "the fertilised egg appears to have equally divided up the three sets of chromosomes into groups of cells, which then split into two, creating the twins", Gabbett said. Sesquizygotic twins were first reported in the US in 2007. The twins came to doctors' attention in infancy after one was identified with ambiguous genitalia. On investigation of mixed chromosomes, doctors found the boy and girl were identical on their mother's side, but shared around half of their paternal DNA.