HOW ARE IDENTICAL TWINS CREATED?
First off, a refresher on identical and non-identical twins. Identical twins share the same genes, whereas non-identical twins only share some of the same genes from each parent – in the same way that siblings do. For these reasons, identical twins are always the same biological sex, while non-identical twins can be different sexes. But, while it’s generally understood that identical twins develop from a single egg (and non-identical twins from two eggs), the finer details of how this egg splits remain somewhat mysterious.
It’s difficult to study splitting inside the human womb without risk to the embryos, but the thinking is that the bundle of cells (or blastocyst) formed from the fertilised egg splits in two during the first week. Exactly when it splits is thought to determine whether the two embryos share a placenta, or each has its own.
Due to the ethical concerns around human embryo research, scientists are working on ways to model the splitting process in the lab using balls of stem cells that behave a bit like blastocysts. While previously these ‘blastoids’ were made from mice stem cells, it has recently become possible to make them from human stem cells. Earlier this year, Dutch and UK researchers published new blastoid-based research supporting the idea that twin pregnancies may be due to rapid expansion of the outer layer of cells in the blastocyst. In fact, chemically boosting the growth of this layer caused more blastoids to split, forming more ‘twins’ in their model.
Interestingly, egg splitting may also occur in a very rare, third type of twin pregnancy: semi-identical twins. This type is thought to be the result of a single egg being fertilised by two sperm before splitting. In this case, the twins may share more genes than nonidentical twins, but can still, it seems, be opposite sexes – as in one of only two confirmed cases.