In Australia, on average 1.4% of pregnancies result in a multiple birth. Multiples result from around 1 in every 80 births, with approximately one-third of twins being identical (MZ) and two-thirds being fraternal (DZ).
Birth rates for MZ twins are consistent among all races (about 4 per 1,000); but the incidence of DZ twinning varies among races (8 per 1,000 among Caucasians, 16 per 1,000 among people of African descent, and 4 per 1,000 among those of Asian descent).
Chances of fraternal twinning
In the case of fraternal twins (clinically referred to as dizygotic twins), the most common factor is an apparent genetic tendency to hyper-ovulate.
Other things like environment, nutrition, age, and weight have also been linked to having twins as well. The rate of multiple birth for women over the age of 35 years is 20%, compared to only 6% in women under the age of 20 years. And there is always simple chance…every woman has a chance at having fraternal twins; it is just that some women have a higher or lower chance.
Fraternal twins occur in about 70% of twin births in Australia.
Chances of identical twinning
The cause of identical (monozygotic or MZ) twinning is still unknown. Research suggests that most cases of MZ twinning are not caused by genetic factors.
However, there have been reports of a few families with a larger-than-expected number of MZ twins, which indicates that genetics may play a role. It is possible that genes involved in cell adhesion (sticking cells together) may contribute to MZ twinning, although this hypothesis has not been confirmed. It’s mostly a spontaneous and scientifically unexplained phenomenon!
Identical twins occur in about 30% of twin births in Australia.
Chances of twinning from IVF and assisted reproductive technology
Contrary to popular belief, the rate of multiple birth from assisted reproductive technology has been on the decline since 2005. Since 2002, the rate of single embryo transfer (SET), where only one embryo is implanted, has increased. Between 2002 and 2014 the rate of SET increased from 30% to 80%, with two-embryo transfer decreasing from 65% in 2002 to 20% in the same period.
A continuing trend in ART treatment in Australia and New Zealand has been the reduction in the rate of multiple deliveries, from 5.6% in 2013 to 3.6% in 2017. This has been achieved by clinicians and patients shifting to single embryo transfer, with the proportion increasing from 76.3% in 2013 to 89.4% in 2017.*
Interestingly, Assisted Reproduction Technology (ART) treatment seems to boost the rate of MZ twinning, but currently, researchers don’t understand why.
* The rate of multiple birth from ART (assisted reproductive technology) is recorded by Australian & New Zealand Assisted Reproduction Database (ANZARD). ANZARD collection is a collaborative effort between the National Perinatal Epidemiology and Statistics Unit (NPESU), the Fertility Society of Australia (FSA) and the fertility centres in Australia and New Zealand. Data of ANZARD is provided by fertility centres in Australia and New Zealand. The NPESU is the ANZARD data custodian for all fertility centres in Australia and New Zealand who provide data to the NPESU.