Doctors Report World’s Second Case of “Semi-Identical” Twins

The extremely rare event, thought to be caused by two sperm cells fertilizing the same egg simultaneously, was last reported in 2007.

Written byCatherine Offord

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Doctors have confirmed that two four-year-old children in Brisbane, Australia, are “semi-identical” twins, making them just the second case of this type of twin ever reported. The boy and girl, described today (February 28) in the New England Journal of Medicine, share identical DNA from their mother’s side, but only some DNA from their father’s side—a situation the authors suspect arose by two sperm cells fertilizing the same egg simultaneously.

“This is confirming there is this third type of twinning where it’s not fraternal and it’s not identical,” study coauthor Michael Terrence Gabbett of Queensland University of Technology in Brisbane tells Reuters Health. “It’s this strange place in between.”

The case was spotted after doctors took ultrasound scans of the mother’s womb. “The mother’s ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins,” study coauthor ...

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Meet the Author

Catherine Offord
After undergraduate research with spiders at the University of Oxford and graduate research with ants at Princeton University, Catherine left arthropods and academia to become a science journalist. She has worked in various guises at The Scientist since 2016. As Senior Editor, she wrote articles for the online and print publications, and edited the magazine’s Notebook, Careers, and Bio Business sections. She reports on subjects ranging from cellular and molecular biology to research misconduct and science policy. Find more of her work at her website.
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