Fertility treatments and the fact pregnancies are happening later in life has made multiple births more common.
Different types of twins
One-third of all twins will be identical and two-thirds non-identical.
Identical twins
Identical (monozygotic) twins happen when a single egg (zygote) is fertilised.
The egg then divides in 2, creating identical twins who share the same genes.
Identical twins are always the same sex, so if your twins are identical, you'll have 2 girls or 2 boys.
Non-identical twins
Non-identical (dizygotic) twins happen when 2 separate eggs are fertilised and then implant into the womb (uterus).
These non-identical twins are no more alike than any other 2 siblings.
Non-identical twins are more common. The babies may be of the same sex or different sexes.
Are you carrying twins?
You might think you're carrying more than 1 baby if:
- you seem bigger than you should be for your dates
- twins run in your family
- you have had fertility treatment
It's usually possible to find out if you're having twins through your 12-week scan.
At the scan, you should be told whether the babies share a placenta (meaning they're identical) or if they have 2 separate placentas (meaning they can be identical or not).
If this is not clear from the first scan, you should be offered another one.
What causes twins?
Nobody knows what causes identical (monozygotic) twins. Everyone has the same chance of having identical twins: about 1 in 250.
Identical twins do not run in families. But there are some factors that make having non-identical twins more likely:
- non-identical twins are more common in some ethnic groups, with the highest rate among Nigerians and the lowest among Japanese
- if you’re pregnant and over 35 you’re more likely to have non-identical twins because you’re more likely to release more than 1 egg during ovulation
- non-identical twins run on the mother's side of the family, probably because of an inherited tendency to release more than 1 egg
IVF can increase the chance of twins, as more than 1 embryo may be transferred.
How can I tell if my twins are identical?
The most accurate way to tell if twins are identical is through a DNA test. This can only be done after your babies are born.
The placenta could also provide clues. If your first ultrasound scan is done before 14 weeks, it should be possible to tell accurately what kind of placenta your twins have.
Otherwise, the placenta can be examined after your babies are born.
For more information on this and birth options, read about giving birth to twins or more.
Read more about the different types of twins on the Twins Trust website.
More detailed information on DNA testing and how to find out whether twins or triplets are identical is available on the Multiple Births Foundation website.