If you are trying to fall pregnant, and would like to know the likelihood of having twins, unfortunately it is not an exact science. There are a few factors that can influence your chances, but a lot of the time, women without any of those factors fall pregnant with twins.
You get two different types of twins. The first being identical, or monozygotic. This occurs when a woman ovulates one egg, which is fertilised and then splits into two parts. Because these twins both come from the same egg and fertilisation, their genes will be shared. The second type of twins is non-identical twins, or dizygotic.
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.
What are the chances?
Depending on where you look, you’ll see estimates for fraternal twins occurring naturally ranging between one in 60 and one in 250 pregnancies. There are thought to be about 125 million twins in the world right now. Triplets and quadruplets are much, much rarer (triplets have about a one in 8000 chance of occurrence, quads a one in 700 000). African fraternal twins are far more common than fraternal twins of Asian or European descent, though, so your chances could be even higher –some sources claim one in 20 births in Nigeria to be twin deliveries. Twins are more common to older mothers (because we women tend to superovulate – that is, release more than one ovum into the uterus during a menstrual cycle – as we age). And because women are having their babies later and later in life, at least in the developed world, it’s becoming tough to pinpoint an exact rate of twin occurrence as this variable changes.
Second and subsequent pregnancies are more likely to result in twins, as are a family history of twin births, especially on the maternal side. Your chance of carrying twins is doubled if you’ve already had a twin pregnancy.
But the single factor that accounts for the greatest increase in twin pregnancies is the use of fertility treatment. Fertility drugs such as clomiphene citrate cause superovulation – and therefore increases the change of multiple pregnancies. And it is standard practice to fertilise and put back more than one ovum during in vitro fertilisation (IVF) – which, if successful, obviously leads to more twin and triplet births. It’s become so common that people these days often assume that any twins they encounter were fertilised in a test tube. Identical twins are a whole different story, though – they occur at a stable global rate of about one in 333 pregnancies.
You may have twins if:
- You have waited longer than usual to have children.
- You have had one or more children before now.
- Twins seem to run in your family.
- You've undergone fertility treatment in order to fall pregnant.