What causes early egg fusion and late sac fusion in multiple pregnancies?

[caters]

I know that if 2 eggs fuse early in pregnancy before a blastocyst forms in either one of them a true hermaphrodite will have formed in a human or animal that is not usually hermaphroditic.

I also know that sometimes in multiple pregnancies, especially in multiple pregnancies that are more than just twins the amniotic sacs will fuse. If they were before all identical and separated early(real early like right after fertilization or something) then for twins you would have dimo(dichorionic, monoamniotic), for triplets trimo(trichorionic, monoamniotic) and so on if the amniotic sacs do fuse late in pregnancy.

But what causes the eggs to fuse early in pregnancy?

What causes the amniotic sacs to fuse late in pregnancy?

 

[Simon Bridge]

Tissues and cells need to be able to fuse together as a normal part of an organisms growth.
As for "cause" - bottom line: it happens because biology is messy.

Also see Chorioamniotic separation for another example of how this process can go wrong.