i found this online
"Unlike the single baby, this fertilized egg cell will split into two separate embryos, and grow into identical twins. 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
•At the early blastocyst stage on days 4 to 6
•Or in the late blastocyst stage on days 7 to 9
The stage at which the egg cell splits determines how the twins will implant in the uterine lining, and whether or not they share an amnion, chorion, and placenta. Basically, the earlier the splitting occurs, the more independently the twins will develop in the uterus. So, a pair of identical twins that split during the two-cell stage will each develop its own amnion, chorion, and placenta.
Twins that split during the late blastocyst stage will share an amnion, chorion, and placenta."