Multiple sets of multiples

Written by Carissa Mason, with input from Shaie O’Brien, Australian Twin Registry

New South Wales mum Caromy McLean was still breastfeeding her first set of twins when she found out she was pregnant with a second set. “I had four in nappies for a year”, Caromy says.

She and husband Andrew are proud parents to two sets of fraternal twins – Hughie and Katie, aged five, and Carys and Holly, aged three. “Life with two sets of twins is pretty challenging”, Caromy says. “I’m kept busy wrangling pre-schoolers while running between swimming, ballet, karate and preschool – not to mention renovating the house we’ve bought.” 

There’s a history of twins in Caromy’s family, with her great-grandmother also having two sets of fraternal twins (both boy/girl sets), born 18 months apart. While Caromy’s situation certainly is rare, she is not alone. The Australian Twin Registry has more than 35,000 sets of twins on its database, representing about 17 per cent of Australia’s 205,000 twin pairs. Its database includes 587 families with two sets of twins. This includes 381 families with two sets of dizygotic (fraternal) twins, 56 families with two sets of monozygotic (identical) twins and 131 families with a set of each. (The other double sets oftwins include at least one set where the zygosity is notknown). Amazingly, the database also shows a handful offamilies with three sets of twins, as well as families withboth a set of twins and a set of triplets.

So do twins run in families? What are the chances of having multiples, well, multiplied? Google will offer youa range of statistics, but credible sources showing clear rates are difficult to find.

In Australia, the rate of twinning is 14.4 sets of twins born per 1,000 confinements, based on Australian Bureau of Statistics figures for 2012 released in October. During 2012, there were 4,480 sets of multiples born across Australia, including 59 sets of triplets or higher order multiples. Worldwide, 4 in 1,000 confinements are identical twins, while rates of fraternal twinning vary among races. It has long been established that fraternal twinning can run in families, with mothers inheriting from either parent the propensity to release two eggs. While a father with this gene could pass it on to his children, it would not influence his chances of having twins. A 1970 study found that a woman with a sister who has fraternal twins was 2.5 times more likely than the general population to have twins. And if a woman’s mother has had fraternal twins, that woman was twice as likely as the general population to also have fraternal twins.

The University of Washington’s Twin Registry website explains a woman’s chances of having fraternal twins also increase with age, fertility treatments, the number of pregnancies she has had and being well nourished. Race and the season also play a role, and large, tall women are more likely to have fraternal twins than small women. Identical twinning, on the other hand, has been widely considered to occur randomly. New research however suggests otherwise. Australian Twin Registry deputy director Dr Jeff Craig, who is also head of the Early Life Genetics group at the Murdoch Children’s Research Institute, explains. “It is emerging that identical twins may, in rare cases, run in families”, Dr Craig says. “We know this because there are isolated communities throughout the world that have a higher than expected number of identical twins. Researchers have already tracked down a gene they have found altered in one particular family”, he says. Developmental biologist Bruno Reversade is examining the genes of villagers in India, in particular in Mohammad Pur Umr, where about one in 10 births are identical twins.

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