The number of twins born in New Zealand has almost doubled since 1980, but twin birth rates have levelled off in recent years as fertility treatments become more refined.
Last year 1774 twins were born in New Zealand - a birth rate of 27.8 twins in 1000 births - compared to 984 twins born in 1980, at a birth rate of 19.5 twins in 1000 births.
The increase is largely because of delays in childbearing combined with fertility treatment, and has been mirrored in other developed countries.
This month the US National Centre for Health Statistics released data which showed the birth rate of twins surged 76 per cent over the past 30 years. It went from a birth rate of 19 twins in 1000 US births in 1980 to 33 per 1000 births in 2009.
However, Statistics New Zealand data shows that twin birth rates in New Zealand since 2000 have stayed at around 27 to 30 twins in 1000 births.
Dr Emma Parry, spokeswoman for the Royal Australian and New Zealand College of Obstetrics and Gynaecology, said the plateau could be explained in part by advances in fertility treatment.
"During the mid-1980s to 2000, fertility treatments were introduced in New Zealand. And a lot of the fertility treatments resulted in multiple births."
In the early days of IVF treatment, two or three fertilised eggs were put back into a patient's uterus to maximise chances of a baby.
But research has since shown that the chances of having a baby are as good if just one embryo is used, which in turn reduces the chance of multiple births.
"There has been a very big shift. And the fertility units in New Zealand are extremely careful in the way that they do it - they're really good units.
"The US units have not tended to take that approach yet, and it's been a lot more financially driven in the US."
Dr Parry said the levelling off in the number of twins being born was good news, as multiple births were more risky for both the babies and mother.
"We're quite pleased about that [the flatline], because they are such higher-risk pregnancies, with more complications."
According to National Women's Hospital's annual clinical report of 2010, the number of babies who died before birth or within the first week of life for single term pregnancies was 13.4 in 1000 births. The rate for multiple pregnancies was 50.3 in 1000 births.
The same report showed that 192 twins were born prematurely at National Women's Hospital last year, compared with 106 twins born at full term.
Last year, 11 per cent of all pregnancies dealt with at Auckland Hospital's fertility clinic, Fertility Plus, were twin pregnancies.
Dr Parry said older women were more likely to give birth to twins, whether fertility treatment had been used or not. However, the mother's age influenced the chances of having non-identical twins only, which develop after two separate eggs are fertilised by two separate sperm.
Identical twins develop after one zygote splits and forms two embryos.
1980 - 984 twins born
1990 - 1374 twins born
2000 - 1714 twins born
2010 - 1774 twins born
NOTE: Births refer to the number of live births. One set of twins is equal to two births.