What causes this phenomenon? In a typical identical twin pregnancy, an egg splits during its first week after fertilization. But in a mirror twin pregnancy, the egg splits 7 to 12 days after it's been fertilized — long enough for the egg to have developed a right and a left side.

At what stage does the egg split for twins?

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).

What causes egg to split in identical twins?

This type of twin formation begins when one sperm fertilizes one egg (oocyte). 1 As the fertilized egg (called a zygote) travels to the uterus, the cells divide and grow into a blastocyst. In the case of monozygotic twins, the blastocyst then splits and develops into two embryos.