You’ve heard of fraternal, and identical twins, but have you ever heard of sesquizygotic twins? University of Queensland researchers have identified what they say is only the second case of this third kind of twinship ever recorded.

March 11, 2019

In pregnancy typically one sperm cell has fertilized one egg, which can either result in one child, or (if the cells divide) identical twins, or if two sperm cells fertilize two eggs, fraternal twins. However, something very rare happened for one Australian family. In this instance the expectant mother had an ultrasound at 6 weeks, which showed one placenta, and the positioning of the amniotic sacs indicated she was expecting identical twins. When she returned for her next ultrasound at 14 weeks however, the scan showed one twin was male, and the other was female, which is impossible in identical twins. Further investigation revealed an extraordinarily rare phenomenon, sesquizygotic twins. In this case the researchers believe one egg cell was fertilized simultaneously by two sperm cells, and then divided which resulted in twins. Only two sets of these kinds of twins have ever been identified.

Source: Sci News