When do identical twins separate in the womb?

The splitting can actually happen at various times and the results will depend on when it happened.

If they split between days 1-3 after fertilization (before implanting or even making it to the uterus from the fallopian tubes) then they will have seperate placentas and seperate amniotic membranes. This is the safest kind of identical twin.

If they split between days 4-8 after fertilization (also before implanting, although the embryos are now blastocysts and have generally made it to the uterus) then they will be monochorionic, diamniotic and will generally share a placenta but will have a membrane between then still.

If they split between days 8 and 13 after fertilization (after implanting into the uterus) they will be monochorionic and monoamniotic and share one placenta and one amniotic membrane. IE - they will be in the same bag of waters together. This is the least safe type as they have a high risk of tangling their cords with each other.

Wiki here states that between days 13 and 15 they could become conjoined twins. The research is still happening in this area so wiki doesn't really know for sure. Some scientists say that conjoined twins are actually just monochorionic monoamniotic twins that stayed too close and refused together.

There are also theories out there for 'polar body twins' or 'half identical' twins. This is basically where the egg splits before being fertilized and then both identical eggs are fertilized by different sperm. The other way for this is if one egg is fertilized by two sperm and then it splits forming chimeras. That's pretty much on the edge of science now though so who knows on that one.

I saw someone said if they have separate sacks they are fraternal. This is not true. Identical twins can have different placentas and sacks if they split earlier rather then later. I know this for a fact. I'm pregnant w triplets right now. A sec of identical twins and one on its own. My doctor knew early on that my twins where identical BC they shared a placenta and sack (which if for the worsted). He told me if they had split later in the cycle there is absolutely no way to tell until they are born or u do testing while they are in the womb BC identical twins have the same DNA make up. So yes they can still be identical. Just in case someone finds this post even though its old I wanted to comment.

How Do Identical Twins Form

How Do Twins Form

The splitting happens in the very first few stages of development. Probably within the first few hours after conception.

within the first few days after conception

after they are fertile,they separate.