How Do I Tell if My Twins Will Be Identical?by Laura Driver
You’ll want to find out if your twins are identical but your obstetrician and midwife will also be keen as the result can affect the type of medical care throughout pregnancy and the birth.
Identical twins always develop from a single fertilised egg that's split and grown into two genetically identical babies. Non-identical (fraternal) twins grow from two eggs, separately fertilised by different sperm.
Defining twins as identical or not is more complicated than you may think. Medical definitions of twins are based on how the twins are conceived - from one fertilised egg or more - and how they are growing in the womb (uterus).
Within the womb, babies are surrounded by two membranes:
- The inner membrane, called the amnion. This membrane forms a bag that contains the amniotic fluid.
- The outer membrane, called the chorion. This membrane forms an extra layer around the amnion. The placenta grows from tissues in the chorion.
By carefully looking at these membranes, and the placenta or placentas, via an ultrasound scan, it's usually possible to tell whether twins may be identical. Your twins can then be placed into one of the following categories:
Dichorionic diamniotic (DCDA)
Each baby has their own placenta and their own separate outer membrane (chorion) and inner membrane (amnion). This is the case for about a third of identical twins and all non-identical twins.
In DCDA identical twins, this happens when the fertilised egg completely splits within three days of conception. This is early enough for the separate membranes and placentas to grow from the egg sac.
So, DCDA twins can be identical or non-identical.
Monochorionic diamniotic (MCDA)
Both babies share one placenta and one outer membrane (chorion), but they each have their own separate, inner membrane (amnion). This is the case for two-thirds of identical twins, so is the most common type of identical twin.
Monochorionic monoamniotic (MCMA)
Both babies share a single placenta, one single outer membrane (chorion) and one inner membrane (amnion). MCMA twins are extremely rare and only account for about one per cent of all identical twins.
When Will I Find Out if my Twins are identical?
If you’re expecting MCDA or MCMA twins, you should find out whether they are identical at your dating scan, between 10 weeks and 14 weeks of pregnancy.
Before 14 weeks, it's easier to see on a scan whether your babies each have their own membranes. Your sonographer will use this information to try to work out if your twins are identical.
If your sonographer identifies DCDA twins, you may not be able to find out for certain whether they are identical, during your pregnancy. You may have to wait until after they’re born to be sure. However, as all non-identical twins are DCDA, and a third of identical twins are DCDA, the chances are they are non-identical.
In addition, some health professionals may assume that two placentas means non-identical twins, when it's possible for identical twins to have a placenta each, if they are DCDA.
If you are pregnant with monochorionic twins, you are more likely to experience complications. Your specialist team will closely monitor you and your babies, and you should have more frequent antenatal appointments and scans.
Though most monochorionic twins are born healthy, up to 15 per cent of twins who share a placenta have win-twin transfusion syndrome (TTTS). TTTS happens when there is an imbalance in the placental blood vessels, meaning one twin gets too much blood and the other gets too little. Your medical team will keep a close watch for TTTS, because it can be very harmful if it's not treated.
Will it be Obvious if my Twins are Identical When They’re Born?
Occasionally it's not possible to tell whether babies are identical twins while they're still in the womb. In that case, you'll only find out for sure after your babies are born. But with newborns, it's difficult to tell just by looking. If your twins are the same sex and each has their own placenta they may or may not be identical. Your obstetrician or midwife may be able to tell by examining the placenta closely.
As your babies grow during their first year, other signs will show if they're identical, such as eye colour, hair colour and texture, skin colour, foot, hand and ear shape, the exact timing and order in which they cut their teeth
If you want to know for sure early on whether your twins are identical, a DNA test is the most accurate way of finding out, but you will have to pay.
The DNA test is called zygosity determination. It establishes whether your twins are identical (monozygotic) or fraternal (dizygotic). You'll just need to sweep a cotton bud inside your twins' mouths. DNA testing isn't usually available on the NHS but is offered by the Multiple Births Foundation for about £100 per multiple tested.