How likely is it that you will have twins? Expectant mothers have a one in 80 chance of having twins. However there are situations where the likelihood increases.

If you are going through fertility treatment or are an older mother your chance of a multiple birth increases. As women get older (35+ years) they tend to produce more eggs during ovulation. Some figures suggest multiple births increase to one in four after IVF treatment.

Changes to the rules on the number of embryos transferred during IVF treatment mean that resulting pregnancies of three or more babies are less likely these days. The Human Fertility and Embryology Authority now permits a maximum of two embryos to be transferred in women under 40 years of age.

There are two types of twins: identical where one fertilised egg divides in half and non-identical where two eggs are fertilised by two sperm. There is believed to be some hereditary link where non-identical twins are concerned and the likelihood also varies from country to country.

The expert view

Patrick O’Brien, spokesman for the Royal College of Obstetricians and Gynaecologists and a consultant obstetrician at the University College Hospital, London, says: “If there are twins in your family it increases your chances of having them marginally. Amazingly if you look at the chances of identical twins it seems to be almost exactly the same across the world — it’s about three per 1000.

But if you look at non-identical twins the incidents change significantly around the world. In Japan it’s traditionally been the lowest at about six per 1000, whereas in some African countries like Nigeria it’s 14 per 1000.

“It might be hereditary but the reason is probably to do with fertility. Non-identical twins result from two eggs being fertilised by two sperm, so the woman has released two eggs at the same time, which is quite a fertile thing to do. The chances of woman releasing two eggs at the same time seem to be different in different countries and then maybe that’s where the familial link comes in. Maybe some families are a bit more fertile, but no one is sure. IVF has a big impact: if one in 80 is the background chance of having twins, it is more like one in 30 or 35 among countries where IVF is more common.”

A mum’s view

Nancy Smith, mother of twins Joseph and Eva, says: “I was seven weeks when I found out I was having twins. I had IVF so they scan you early to make sure they’ve taken. I knew there was a greater chance of having twins but I still thought it wouldn’t happen! There were twins in the family – my sister has them – so they did say there was more chance. Because of that the doctors had a consultation about whether or not to put two eggs back.”