Zygotic splitting occurs between days two and six when the zygote divides, usually into two, and each zygote then goes on to develop into an embryo, leading to identical twins (or triplets if it divides into three). These are known as "monozygotic" twins (or triplets).

How late can an embryo split into twins?

In 5% of the MZT cases, the splitting occurs 8-13 days after fertilization, resulting in monochorionic-monoamniotic twins (one placenta and one amniotic sac). Rarely, the splitting occurs after 13 days, resulting in conjoined twins.

Can a blastocyst turn into twins?

Because of the high implantation potential of blastocyst embryos, we should be cautious about the high potential for twins. Transfer of two good looking blastocysts can result in twin pregnancy rates as high as 50%.