Fertility treatments are seen as the main reason for the steady increase in twin births in New Zealand from 1980 until recent years, when the numbers have leveled off as treatments have presumably become more refined to reduce the probability of conceiving twins.

Live birth statistics show that the number of twins born increase by around 20% every 10 years since 1980. In 2010, however, the increase has dwindled and leveled off the previous years' surge in twin births.

There were 1774 recorded birth of twins in the country in 2010, compared to 984 in 1980, 1374 in 1990, and 1714 in 2000.

The birth of twins is counted once in the statistics, which means the numbers are reported in pairs.

The New Zealand Herald reported the increase is largely because of delays in childbearing among Kiwi women, considered together with their use of fertility treatments.

This scenario is mirrored in other developed countries.

Just this month, the US National Centre for Health Statistics reported the birth rate of twins surged 76 per cent over the past 30 years, with a birth rate of 19 twins in 1000 US births in 1980 to 33 per 1000 births in 2009.

In contrast, Statistics New Zealand reported that the rate of twin births in New Zealand has remained 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 notable leveling off the numbers could be partially attributed to technology advancements in fertility treatments.

"During the mid-1980s to 2000, fertility treatments were introduced in New Zealand. And a lot of the fertility treatments resulted in multiple births," she said.

When IVF (In vitro fertilization) has just been introduced, the procedure typically involved two or three fertilised eggs (embryos) being placed into a woman's uterus to maximise chances of a baby.

But studies have since shown that a woman's chances of having a baby do not significantly increase with the placement of an extra embryo.

"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 doctor also said the dwindling of the number of twins born was good news, considering that multiple births were in fact more risky and could be highly complicated for both the babies and mother.

"We're quite pleased about that [the flatline], because they are such higher-risk pregnancies, with more complications," Dr. Parry told the Herald, adding that older women were more likely to give birth to twins, whether fertility treatment had been used or not.

The Herald reported data from National Women's Hospital's annual clinical report of 2010, in which 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, while the rate for multiple pregnancies was 50.3 in 1000 births.