In case of identical twins, when does the blastomeres of a zygote gets separated(before or after fertilization)?

Dear student,In case of identical twins, an egg is fertilized by only one sperm but then the zygote formed divides into two separate embryo.This remarkable event takes place during the first week after fertilization, and can happen at several different times: At the two cell stage on days 1 to 3 or At the early blastocyst stage on days 4 to 6.