Interesting Facts About Twins

There are approximately 400,000 twins in Australia or 2% of the total population.  (Around 17% of all twins are members of the Australian Twin Registry).

Around one-third of all twins are identical and the other two-thirds are non-identical or fraternal.

The number of multiple births in Australia has increased by 27% in the 20 years from 1989 to 2009.  In 2009, there were 4444 multiple births. Of these, 4,358 or 98% were twins, and 86 or 2% were triplets and higher order multiples (HOMs refers to quadruplets, quintuplets and higher).

While the 2009 number of multiple births was 27% higher than in 1989, it was 3% lower than the number in 2008. This 3% decrease in 2009 compared with 2008 can be explained in part by trends in assisted reproduction technology (see details below). 

Of the total 291,240 births in Australia in 2009, 4358 were twins or 1.5% of total births and 86 were HOMs or .03% of total births.

The growing number of multiple births in the past two decades is due largely to the increased use of fertility drugs and assisted reproduction technology, the delay in childbearing and the growing number of older mothers.

An increasing number of babies today are being born with the aid of assisted reproduction technology (ART), which uses medical technology such as in-vitro fertilization or other fertility treatments to assist in the conception of a child.

In 2008, an estimated 3.3% of all births in Australia were the result of ART treatment.  Between 2004 and 2008, the number of births occurring in Australia and New Zealand as a result of ART treatment increased by 47.8%.

However, a clear trend in ART treatment over the past five years has been the reduction in twin and triplets births, with the multiple delivery rates falling from 16.4% in 2004 to 8.4% in 2008.   This trend is due to an increase in the proportion of ART treatment cycles using single embryo transfer, from 40.5% in 2004 to 67.8% in 2008.

Around one in three sets of twins is identical. This occurs because the fertilized egg divides in two while it is still a tiny collection of cells.  The self-contained halves then develop into two babies, with exactly the same genetic information.  Twins conceived from one egg are called identical or (one cell) ‘monozygotic’ twins.

Around two in three sets of twins are fraternal.  Two separate eggs are fertilized by two separate sperm, resulting in fraternal or ‘dizygotic’ (two cell) twins.  These babies will be no more alike than singleton siblings born at separate times, hence the term ‘fraternal’ meaning brotherly.  The babies can be either the same sex or different sexes, with the odds roughly equal for each.

The breakdown of multiple births by state in 2009 was:  NSW 91,424 total births with 1310 twins and 28 HOMs; Victoria 69,745 total births with 1133 twins and 21 HOMs; Queensland 65,061 total births with 1014 twins and 19 HOMs; Western Australia 30,442 births with 435 twins and 8 HOMs; South Australia 19,445 total births with 283 twins and 4 HOMs; Tasmania with 6550 total births with 59 twins and 3 HOMs; Northern Territory 3765 total births with 53 multiple births (twin/HOM breakdown not available); ACT 4785 total births with 74 multiple births (twin/HOM breakdown not available).