One egg, two sperm: Rare twin type between identical and non-identical twins

According to a study published by The Journal of Human Genetics that coined the term and reported its very first case in the US in 2007, semi-identical twinning was described as being somewhere between identical and non-identical twins. Only two cases of semi-identical twinning have been recorded thus far, the most recent from Australia being reported in The New England Journal of Medicine in 2014. A 28-year-old primigravida at the time (2014) underwent a routine ultrasound scan at six weeks showing a single placenta and positioning of the amniotic sacs suggestive of a monozygous pair. However, a subsequent scan at fourteen weeks showed the fetuses were different genders, making it impossible for them to be identical twins.

Aside from identical (monozygotic), non-identical (fraternal/dizygotic), the lesser understood conjoined and polar body (half-identical), scientists have discovered a different kind of twinning: semi-identical or sesquizygotic twins.