How to know if your twins are identical or fraternal

So, you're having twins! But do you know if they are identical or fraternal? Find out...

Will my twins be identical or fraternal?

Discover whether you're expecting identical or fraternal (non-identical) twins, the science behind twin pregnancies and increasing your chances!


Q & A: Will my twins be identical or fraternal?

If you’re hoping for or carrying twins you’re probably very curious to know if they’re going to be identical or non-identical (fraternal)! Approximately 1.54% of all births in the UK are recorded as multiple births including twins, triplets and more, and you should find out if you’re carrying during your dating ultrasound scan.

Before then here’s all you need to know about what your chances are of having twins and what type of twins. There are 3 types of twins (and these apply to triplets too):

  • Dichorionic diamniotic (DCDA) twins -each has their own placenta with its own separate inner membrane (amnion) and outer membrane (chorion)
  • Monochorionic diamniotic (MCDA) twins – share a single placenta with a single outer membrane and 2 inner membranes 
  • Monochorionic monoamniotic (MCMA) twins – share both the inner and outer membranes.

    All non-identical twins, and a third of identical twins are DCDA. 

The other two-thirds of identical twins are MCDA, and just 1 in 100 identical twins are MCMA. 
One-third of all twins will be identical and two-thirds non-identical.

Twins and your dating ultrasound scan

Between 8 - 14 weeks of pregnancy you’ll have your dating ultrasound scan where your sonographer will have a first peek and identify if you’re expecting multiple babies. If you are, the next step will be to identify the chronicity of your pregnancy - whether or not your eggs are sharing the same placenta (monochorionic) or if they have their own separate placentas (dichorionic).

If your eggs are sharing a placenta you’re expecting identical twins! If they have separate placentas it’s nearly certain that the will be fraternal (non-identical) twins, but it is still possible they might turn out to be identical as 30% of identical twins don’t share a placenta.

Fraternal (non-identical) twins

There are a number of factors we know increase the likelihood of giving birth to fraternal twins:

  • IVF (in vitro fertilization) where more than one embryo is transferred
  • Family history of non-identical twins on the mother’s side
  • Older mothers are more likely to have fraternal twins 
  • Fraternal twins are more common in some ethnic groups (highest rate among Nigerians) and less common in others (lowest rate among Japanese).
  • Fraternal (non-identical) twins are produced when two separate eggs are fertilised by two different sperm. Two zygotes are formed and your two twins will develop at the same time in the womb. Because two separate eggs are fertilised they have separate genetic profiles and any physical look-a-likes or differences will occur just like any other siblings!

Identical twins

What exactly causes identical twins to form remains a mystery, however there are some facts we do know that effect having identical twins:

  • Every pregnant woman has approximately a 0.4% chance of having identical twins. 
  • About one third of multi-pregnancies will result in identical twins. 
  • Family history doesn’t increase or decrease the likelihood of identical twins 
  • Identical twins will always both be of the same sex.

You can produce identical twins when a single egg is fertilised by a sperm to form a zygote, which then goes on to divide into two separate embryos in the womb. Identical twins are genetically extremely similar and as such will typically look very alike - identical even!

How to know if your twins are identical or fraternal