What types of twins are there?

Twins can be divided into different types based on how they are formed and how they share resources in the womb. Here are the two main types of twins:

1. Conjoined twins (dizygotic twins)

  • Formation: Conjoined twins are formed when two separate eggs are fertilised by two different sperm cells. Each foetus develops from its own egg and sperm.
  • Genetics: Fraternal twins share approximately 50% of their genes, which means they are no more genetically similar than normal siblings. They can be of the same or different sex.
  • Placenta and amniotic fluid: Each twin has its own placenta (dichoric) and amniotic membrane (diamniotic).

2. Identical twins (monozygotic twins)

  • Formation: Identical twins occur when a single fertilised egg splits in two during the early stages of development. The two parts develop into separate foetuses.
  • Genetics: Identical twins have identical DNA, which means they are genetically identical and are always the same sex.
  • Placenta and amniotic fluid: Depending on when the egg splits, identical twins can have different combinations of placenta and amniotic membranes:
    • Dichoric-diamniotic (DCDA): If the division occurs within the first three days after fertilisation, each twin will have its own placenta and amniotic membrane, just like fraternal twins.
    • Monochoric-diamniotic (MCDA): If the division occurs between days 4 and 8 after fertilisation, the twins will share a placenta but have separate membranes.
    • Monochorionic-monoamniotic (MCMA): If the division occurs between days 8 and 13, the twins will share both placenta and amniotic membranes, which is rarer and can cause more complications.
    • Conjoined twins (Siamese twins): If the division occurs later than day 13, the twins cannot be completely separated, resulting in them being physically joined.

But are they identical?
Even if you were told during your pregnancy that your twins are fraternal, this isn’t necessarily the final reality (unless they are separate sexes).

Twins who are found to be fraternal in pregnancy but are of the same sex can be monozygotic, but it’s not always possible to tell on a scan. These are dichorionic-diamniotic twins, where the egg has split so early that the foetuses have developed separate placentas and amniotic membranes.

Determining that they are fraternal twins also affects the further course of healthcare, as a fraternal twin pregnancy can be less risky than a monozygotic pregnancy. Therefore, it is not important to the midwife whether they are identical or not, only that they do not share membranes or placenta.

If you are in doubt when your twins are born, it is possible to have a (self-paid) DNA test done.

To summarise
Twins can be fraternal (dizygotic) or identical (monozygotic), and within identical twins there can be four different configurations depending on when the egg splits.

Each type of twin pregnancy has its own unique characteristics and potential risks, making close monitoring of the pregnancy essential.