What causes twins
In 2010, more than 11,100 sets of twins and over 175 sets of triplets or more were born in England and Wales. About 1 in every 65 pregnancies in the UK today is a twin pregnancy, which means that about 1 in 32 babies born is a twin.
This is a steep rise from 30 years ago, when 1 in 52 babies was a twin. A couple is more likely to have twins if there are twins in the woman's family. Triplets occur naturally in 1 in 10,000 pregnancies, and quads are even rarer. Nowadays, the use of drugs in the treatment of infertility has made multiple births more common.
There are two types of twin pregnancy.
Identical (monozygotic) twins happen when a single egg (zygote) is fertilised. The egg then divides into two, creating identical twins who share the same genes as one another. Identical twins are always the same sex, so if your twins are identical you'll have two girls or two boys, and they'll look very alike.
Non-identical (dizygotic) twins happen when two separate eggs are fertilised and then implant into the woman’s womb (uterus). These non-identical twins are no more alike than any other two siblings. Non-identical twins are more common. The babies may be of the same sex or different sexes. One-third of all twins will be identical and two-thirds non-identical.
Are you carrying twins?
You might suspect that you are carrying more than one baby if:
- you seem bigger than you should be for your dates
- twins run in your family
- you have had fertility treatment
It is usually possible to find out if you're having twins through your dating ultrasound scan, which happens when you are 8-14 weeks pregnant.
You should now be told whether the babies share a placenta (meaning they are identical) or if they have two separate placentas (meaning they can be identical or not). If this is not determined, you should be offered another scan. One-third of identical twins have separate placentas. This occurs when the fertilised egg splits before implanting in the uterus, up to four days after conception.
What causes twins?
No one knows what causes identical (monozygotic) twins. It appears that all women, irrespective of ethnicity, have an equal chance of having identical twins and that the chance is approximately 1 in 350-400. Identical twins do not run in families.
Some factors make non-identical twins more likely:
- non-identical twins are more common in black populations
- the chance of having twins rises with age and is higher the more children you have already had
- non-identical twins run on the mother’s side of the family, but probably not the father’s
IVF (in vitro fertilisation) and assisted conception involve implanting multiple embryos. After natural conception, about 1 in 80 births in the UK results in multiples, compared to one in four after IVF. Women getting pregnant when they’re older is also a factor, as the rate of twins increases with maternal age. The rate of twins in pregnant women of different ages is:
- 6.3% in mothers under 20
- 21.7% in women who are 35-39 years old
- 56.7% in the over-45s
How can I tell if my twins are identical?
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 placenta could also provide the answer. If your first ultrasound scan is done before 14 weeks, it should be possible to tell accurately the type of placenta your twins have. Otherwise the placenta can be examined after the birth. However, neither of these methods is foolproof.
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 (amnion) and outer membrane (chorion). These types of twins are called dichorionic diamniotic (DCDA).
Two-thirds of identical twins have a single placenta with a single outer membrane and two inner membranes. These are called monochorionic diamniotic (MCDA). About 1% of monochorionic twins will also share the inner membrane. These are called monochorionic monoamniotic (MCMA).
More detailed information on how to find out whether twins or triplets are identical and on the DNA test is available on the Multiple Births Foundation website.
It is possible to breastfeed twins and even triplets. You may find that a combination of breast and formula feeding is best for you, particularly if you have triplets or more. You might find it helpful to contact support groups like TAMBA (Twins and Multiple Births Association) and the Multiple Births Foundation before your babies are born.
Myths about twins
There are many myths about twins. Here, we separate fact from fiction.
Myth: Twins run in families
Fact: There is no evidence that identical twins run in families. However, it is possible that a family could have a genetic predisposition to non-identical twins, probably due to the woman having a predisposition to releasing more than one egg each month when she ovulates.
Myth: Twins skip a generation
Fact: It is a common misconception that twins skip a generation in families. You may have heard, for example, that if your father is a twin but you're not, you're more likely to have twins yourself. There's no evidence to support this. However, some women may have a genetic predisposition to hyperovulation, where they produce more than one egg during a menstrual cycle. This makes it more likely that they will have twins. If this is the case, twins are just as likely to be born to each generation as they are to skip one.
Myth: Having lots of morning sickness means you’re pregnant with twins
Fact: Not necessarily. Although some mothers expecting multiple births report lots of morning sickness, other mothers who are pregnant with twins or more don’t experience any morning sickness. Some pregnant women have nausea and vomiting and some don’t, whether or not they’re carrying twins.
Myth: Twins speak their own secret language
Fact: Twins have an innate understanding of each other and, as a result, may speak in their own code. Also, because they spend so much time together, one may pick up words said wrongly by the other twin that they both understand, which can be perceived as a twin language by other people.
Myth: All pregnancies start out as twin pregnancies
Fact: It isn’t true that all pregnancies start out as twin pregnancies. However, thanks to early pregnancy scans, it has been discovered that more pregnancies than we thought do start out with two fertilised eggs.
It's possible that if you're scanned before 12 weeks, you'll see two foetal heartbeats and two foetal sacs, but one will have disappeared by the 12-week scan. This is because one of the embryos failed to thrive and it has been reabsorbed into the womb. This is referred to as vanishing twin syndrome and has no physical effect on the surviving baby.