The twins are now four-years-old - and technically semi-identical.

Doctors have discovered a set of twins who were born from one egg and two different sperm. The twins are one boy and one girl, who have identical DNA from their mother, but different DNA from their father. This is the second time that a pair of "semi-identical" twins have been discovered. Mail Online reports that the four-year-olds, who live in Brisbane, Australia, were fertilised naturally by two separate sperm from their father and one egg from their mother. Doctors initially thought that twins were identical following an ultrasound scan where they were sharing the same placenta. However, they eventually realised that the twins were different sexes - meaning that it would have been impossible for them to be identical. Semi-identical twins are extremely rare. Scientists believe that most foetuses growing this way do not survive outside of the womb. The unnamed four-year-olds, however, are said to be "beautiful and healthy." This comes just over a month after twins born through an advanced IVF treatment had two different biological fathers.  Two men from London travelled abroad to receive the treatment and were able to successfully fertilise two embryos in their surrogate's womb. The babies were technically half-siblings, despite sharing a uterus and being born at the same time.