A pair of twins have stunned researchers after it emerged that they are neither identical nor fraternal – but something in between.

The team say the boy and girl, now four years old, are the second case of semi-identical twins ever recorded, and the first to be spotted while the mother was pregnant.

The situation was a surprise to the researchers. An ultrasound of the 28-year old mother at six weeks suggested the twins were identical – with signs including a shared placenta. But it soon became clear all was not as it seemed.

“What happened was the mother came back for her routine ultrasound some months later, and we saw one [twin] to be a boy and one to be a girl,” said Dr Michael Gabbett, first author of the report from Queensland University of Technology in Australia. “At that point we started the genetic studies and worked it out from there.”