What causes the fertilized egg splitting into two zygotes when identical twins occur? ?

Question

What causes the fertilized egg splitting into two zygotes when identical twins occur? ?

Answer 1

Chance. And that's pretty much it.

See, when the zygote undergoes the first round of cell replication, it usually stays together. But at this point, when it's just a couple of cells, it can split in such a way that it forms 2 separate beings (just like amoebas and other asexually reproducing organisms do it) instead of staying together as one. This is rare, but it does happen very early on in development.

Answer 2

I'm the mother of identical twins and it's something that happens randomly in about 1 out of every 250 pregnancies.  However, there is new research that is suggesting there might be a biological tendancy to have identical twins because there are some families which have a higher than normal rate of identical twins.

Hope this helps!