Twins born in Australia have become the second-ever-identified pair of “semi-identical” twins, and the first to be detected before they were born. Such cases come about when twins receive 100% identical genes from their mother, but not from their father. The research was published in the New England Journal of Medicine.

At six weeks into the pregnancy, doctors found on the mother’s ultrasound that she was expecting identical twins. However, eight weeks later the children were identified as male and female, an impossibility for identical twins.

Both infants possess a mixture of cells, some with XX and some with XY chromosomes. Unlike the first pair of semi-identical twins, neither of these new twins physiologically present as intersex.

A possible explanation for this genetic confusion is that the mother’s ova might have copied itself prematurely before being fertilised by two sperm, but failed to fully separate. Alternatively, the mother’s egg could have been fertilised simultaneously by two of the father’s sperm before dividing, splitting up the three sets of chromosomes into groups of cells, which then divided further.