These Australian twins are among the rarest ever documented

A pair of Australian twins are among the rarest human beings ever documented, according to a study released Thursday.

The brothers and sisters of Brisbane – a boy and a girl – were classified in the second case reported to the world of sesquizygotic, or semi-identical, twins, and the first to be identified while still in the womb of their mother, according to the paper published in the New England Journal of Medicine.

The sesquizygotic twins, genetically, are somewhere between fraternal and identical twins.

Michael Gabbett, a geneticist at the Queensland University of Technology, and one of his co-authors, Nicholas Fisk, Obstetrician and Vice President of Research at the University of New South Wales, oversaw the Fetal care of twins in 2014 at the Royal Brisbane and Women's Hospital.

At the beginning of the second quarter, it became apparent that there was something unusual about them.

"They are 100% identical on the mother's side and 78% of the [on] on the fathers' side, that equals on average 89%, "Gabbet told Gizmodo.

According to Fisk, it is thought that the twins – now four years old – were formed when an egg was fertilized simultaneously by two sperms.

"The mother's ultrasound at six weeks showed only one placenta and the positioning of the amniotic sacs indicating that she was expecting identical twins," he said in a statement. "However, an ultrasound at 14 weeks showed that the twins were men and women, which is not possible for identical twins."

Fisk immediately called Gabbett about the surprising results.

"I was a little cheeky and I accused Professor Fisk of having insufficient ultrasound skills, but it turns out that they were perfectly good," Gabbett said. at the Australian Broadcasting Corporation. "It was a very unusual genetic event."

It's possible that double fertilization will occur, but these embryos usually do not survive, Gabbett said at the exit.

The family refused to be identified, but the twins are doing well and are reaching all their development milestones, according to the doctor.

"They look very similar," he said at the exit. "They are very cute and they are doing very well."

The researchers examined genetic data from 968 other fraternal twins and their parents – but found no set of sesquizygous twins.

"It was a very unusual genetic event," Fisk told ABC, noting that only one other case had been reported – in the United States in 2007.