• Doctors in Australia say they have identified a pair of rare “semi-identical” twins.
• Their report was published Thursday in the New England Journal of Medicine.
• Genetic testing revealed that the twins, who are now 4 years old, share all of their mother’s DNA but only a portion of their father’s.
• One of the report’s authors said it’s likely that a single egg from the mother was fertilised by two sperm from the father before dividing in half.

Doctors in Australia say they have discovered a rare set of twins who are neither fraternal nor identical, but something in between.

In a report published Thursday in the New England Journal of Medicine, the researchers said they used prenatal genetic testing to identify a set of ‘semi-identical’ twins, USA Today reported. According to a press statement about the research, the twins are only the second set of semi-identical twins ever reported in the world, and the very first set to be identified while still in utero.

Identical twins are formed when a single egg is fertilised by a single sperm and then splits, resulting in twins who share the exact same genes.Fraternal twins occur when two separate eggs are fertilised by two separate sperm, resulting in twins who share 50% of their DNA.

But scientists have postulated that there’s a third type of twins, the authors of the report wrote: Semi-identical, or sesquizygotic, in which twins share between 50 and 100% of their DNA.

The twins shared a placenta, but one was male and one was female

In the report, the authors described the case of a 28-year-old woman who was pregnant with twins. Six weeks into the pregnancy, she underwent an ultrasound that showed her twins were sharing a placenta, an indication that they were identical.

“However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins,”Nicholas Fisk, a professor at the University of New South Wales and co-author of the report who helped care for the mother, said in the statement.

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Further genetic testing of the twins’ amniotic fluid revealed that they were maternally identical but shared only part of their father’s DNA, the report concluded.

“It is likely the mother’s egg was fertilised simultaneously by two of the father’s sperm before dividing,” Fisk said in the statement.

Evidence suggests this is a rare phenomenon

The authors wrote that there’s one previous report of semi-identical twins, published in 2007. In that case, the twins also shared 100% of maternal DNA but only part of their paternal DNA.

The authors also performed genetic tests of 968 sets of other twins presumed to be fraternal to see if any were really semi-identical. They found that none were.

“We know this is an exceptional case of semi-identical twins,” Fisk said in the statement.

The twins are now 4 years old

Michael Gabbett, a geneticist at the Queensland University of Technology and another author of the report, explained in the statement that embryos “do not usually survive” in a situation where one egg is fertilised by two sperm, creating three sets of chromosomes.

“Three sets of chromosomes are typically incompatible with life,” he said in the statement.

But in this case, the twins did survive and were delivered by cesarean section at 33 weeks of pregnancy, according to the report.

After birth, one twin experienced a blood clot that required the amputation of one arm, the authors wrote. Three years later, doctors discovered that the same twin had a condition called gonadal dysgenesis, a condition affecting the development of the body’s sex organs. Otherwise, the report said, both children were developmentally normal.

The report also included a video with some helpful animations to illustrate how semi-identical twins occur. Watch it below:

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