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One in every 32 babies born in the UK today is a twin, mainly thanks to the increased use of fertility treatments.  

Around 12,000 sets of twins and 130 sets of triplets are born in the UK every year, this means that around one in 32 of all babies born is a twin, a steep rise from 30 years ago when the figure was only one in 52 babies.

The increased rate of twin pregnancy is largely attributable to the rise in IVF and assisted conception, which involve implanting multiple embryos. After natural conception about one in 80 births in the UK are multiples, compared with one in four after IVF. Rising maternal age is also a factor as the twinning rate increases from 6.3 percent in mothers under 20 to 21.7 in 35-39 years, and 56.7 in the over 45s.

Types of twin pregnancy

There are two different types of twin pregnancy:

• Identical (monozygotic) twins
  This type of multiple pregnancy occurs when a single egg (zygote) is fertilised. The egg then divides into two creating identical twins who share the same genes as one another.
• Non-identical (dizygotic) twins
  This type of multiple pregnancy occurs when two separate eggs are fertilised and then implant into the woman’s uterus. These non-identical twins are no more alike than any other two siblings.

What causes twins?

No one knows what causes identical (also called monozyogotic) twins. It appears that all women, irrespective of ethnicity,  have an equal chance of having identical twins and that chance is approximately one in 350-400. Identical twins do not run in families.

There are known predisposing factors when it comes to non-identical (also called dizygotic) twins:

• They are more common in black populations, while it has been reported by some studies that rates are lower for Koreans, Thais, Vietnamese and Japanese. Rates for white populations and Asian Indians are somewhere in the middle.
• The chance of having twins rises with age until the late 30s and is higher the more previous children you have had.
• Non-identical twins run on the mother’s side of the family, but probably not the father’s.

How can I tell if my twins are identical?

All non-identical twins, and one third of identical twins have exactly the same type of dichorionic placenta; this is when each baby has its own separate placenta with its own separate inner membrane (the amnion) and outer membrane (the chorion). This type of twins is called dichorionic diamniotic (DCDA).

Two thirds of identical twins have a single placenta with a single outer membrane and two inner membranes and are therefore called monochorionic diamniotic (MCDA). About one percent of monochorionic twins will also share the inner membrane and are therefore called, monochorionic monoamniotic (MCMA).

If the first ultrasound scan is done before 14 weeks it should be possible to make an accurate diagnosis of the type of placenta your twins have. Or the placenta can be examined after the birth.  However, neither of these methods is foolproof.

The most accurate way to tell if twins are identical is through a DNA test, which can be done after the babies are born.

The Multiple Births Foundation produces a useful booklet which explains the test called Are they identical? Zygosity Determination for twins, triplets and more.