Are there four types of twins?

[typesoftwins]

Is it correct that there are 4 types or am I wrong?

1. fraternal twins from 2 separate eggs and sperm
2. Identical twins where the embryo splits extremely early and they separate (so looks like fraternal twins on early scan)
3. identical twins in same sac with separate placentas
4. identical twins in same sac with same placenta and membranes?

I think that’s right

Thankyou ! Was just having a discussion with a friend who didn’t believe me about no.2 being possible !

I don't think 2 is possible. For that to happen, both the embryo and the extra embryonic tissues would have to split independently and then both 'halves' successfully implant separately.

Embryos can split more easily as it happens typically at the 2-4 cell stage. Embryo cells are not differentiated at all, so they can still form a normal baby. Extra embryonic cells on the other hand are differentiated and can't substitute for each other in the same way.

This is what my friend is saying and now it’s really annoying me that I don’t know !!!

This is a nice picture that shows types.

And this explains how they form. But yes identical can be totally separate sacks and placentas if they split early. https://multiples.org.nz/types-of-multiples/more-about-different-types-of-twins/

And this explains how they form. But yes identical can be totally separate sacks and placentas if they split early. https://multiples.org.nz/types-of-multiples/more-about-different-types-of-twins/

The fertilised egg splits one or more ways to create separate embryos.

I have triplets - a set of identical twins who had their own sac and placenta as the egg split very early and a singleton. So, your no.2 type.

With these type of twins, DNA testing gives confirmation.

Sounds right, I've got the first type but was told as same sex wouldn't know if identical or not until they were born, although it was far more likely to be non identical. One had blue eyes the other brown when born so that was quickly settled.

You can have number 2. 25% of DCDA TWINS (different sack and placenta) are identical

https://www.bbc.co.uk/news/health-47371431

There's a rare type of semi-identical twins as well. 1 egg simultaneously fertilised by 2 sperm.