This is how twins are created


How are twins made? We chatted to Dr Sulaiman Heylen, a specialist in reproductive medicine at the Cape Fertility Clinic, to find out a bit more about the fascinating process that results in multiples


According to Dr Heylen, there are two types of twins – non-identical and identical. “Fraternal twins,” he explains, “are nonidentical twins which means they are each created from a different sperm and egg.”

In other words, you can sometimes ovulate twice in the same month and could simply strike it lucky if your partner’s sperm – on one or two separate occasions – fertilises both of those eggs that have been released in the same cycle. It’s all up to you and your body, though, because, “there are no known contributing factors from the male for the incidence of non-identical twins,” he maintains.

While conceiving twins might just be a complete fluke, Dr Heylen notes that the chances of having them increases significantly in women between 35 and 40, tall women and those women who already have several children. 

“Non-identical twins are also more common in certain ethnic groups,” he says, adding that while the highest incidence of twins happens in West Africa and South America, and the lowest incidence of twins occurs in South Asia and Japan.

“Fraternal twins are also called dizygotic twins and there is a 50 percent chance of your dizygotic twins being a girl and a boy, a 25 percent chance of them being two boys, and a 25 percent of them being two girls.”


Identical twins, or monozygotic twins, are less common,” says Dr Heylen, and they occur when a single egg is fertilised to form one zygote that then divides into two separate embryos.

“They are genetically nearly identical and are always two boys or two girls. The incidence is around 3 per 1000 births and is uniformly distributed in all ethnic groups around the world,” he explains.

Identical twins have more complications than non-identical twins and there is the risk of twin-to-twin transfusion syndrome. “As a result of sharing a single placenta (non-identical twins each have their own placenta), the blood supplies of monochorionic twin babies can become connected, so that they share blood circulation. What then happens is that one of your babies receives too much blood and the other too little.”


Some women who struggle to fall pregnant may be prescribed fertility drugs such as Clomid or Femara which stimulate the ovaries so that more eggs are released, increasing the chances of a twin pregnancy.

Of course, if you underwent IVF treatment and chose to have two fertilised embryos put back into your uterus, that would explain your multiple pregnancy. This is something you would chat to your doctor about if and when the time comes.

“These days we should not transfer more than two embryos. The question is now if we transfer one or two embryos, because even though most twin pregnancies are normal and most women will deliver two healthy babies, a twin pregnancy is still a high-risk pregnancy and the risk of early labour, pre-eclampsia and pregnancy diabetes increases when you’re carrying two or more babies,” stresses Dr Heylen.

“It is wrong in nearly all cases to transfer three or more embryos. High order multiple pregnancies (more than two babies) are very dangerous for the babies and to a lesser degree for the moms. All multiple pregnancies have an increased risk of long term neurological problems too.”

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