Multiple births are becoming more common because of fertility treatments and the fact that women are waiting later to have their babies. In the UK, twins happen in about 1.5% of pregnancies. This is a big increase from 1984, when 1% of every birth was a multiple birth.
Research has proven that tall women are more likely to have non-identical twins. This is because taller women apparently have more insulin-like growth factor (IGF). This is a protein that is released from the liver to stimulate growth in the shaft of longer bones. Higher levels of IGF results in increased sensitivity of the ovaries, thus increasing a woman’s chance of ovulating. The research, which was performed by Dr Gary Steinman from Long Island Jewish Medical Center, says that the more IGF a woman has, the greater chance she has of becoming pregnant with twins, because IGF “governs the rate of spontaneous twinning”. In a previous study, Dr Steinman also found that women who eat dairy are five times more likely to have twins. This has been put down to levels of IGF in cow’s milk.
In total, around 12,000 sets of twins are born in the UK every year. Non-identical (or fraternal) twins are more common. Two-thirds of all twins are non-identical and one-third are identical.
Non-identical twins are the result of separate fertilised eggs. These babies are no more alike than any other brothers or sisters, and may be both male, both female, or one of each. They share DNA in common as in the case of siblings from different births. Identical twins are formed from a single fertilised egg, which went on to split into separate embryos. This means their DNA is exactly the same. Zygosity testing, or twin testing, is used to determine whether multiple children from the same birth are genetically identical or not. This twin DNA test only needs cheek buccal cells so samples can be easily and painlessly collected.
There’s no evidence that being from a family with identical multiples has any impact on the odds of having identical twins. Non-identical twins, on the other hand, do run in families. It’s been proven that heredity on the mother’s side double the chances of a couple conceiving non-identical twins. This is because a certain gene predisposes some women to hyperovulation where more than one egg is released during each menstrual cycle. If a man inherits the hyperovulation gene from his mother, he may pass this gene on to his daughter. His daughter, in turn, is then more likely to release more than one egg when she ovulates and therefore could conceive non-identical twins. In this case, the gene is still passed down the mother’s side but the twins have skipped a generation.
So, in summary: your odds of conceiving twins are improved if you have a history of non-identical twins already in your family, if you are an older mother, if you are undergoing fertility treatment, if you’re taller than average, and if you are fond of dairy. More facts about twins can be found on our Learning Centre article ‘Fun facts about twins’.