What's behind New Zealand's low fertility rates?

More babies are born to mothers in Gisborne than any other part of Aotearoa, Stats NZ says.

Every month, Stats NZ joins RNZ's Nights to find out what statistics can tell us about life in New Zealand.

On Wednesday, population insights analyst Rebekah Hennessey discussed birth and fertility rates and what birth data was used for.

Hennessey told Nights the average amount of births for women in Gisborne was 2.3. That compared to 1.75 for the whole of New Zealand.

A higher proportion of women in the region were having babies at a younger age, whereas 40 percent of people having babies in New Zealand were over the age of 30.

Over the last decade, Hennessey said the number of births each year was tracking down - but it had not actually changed a great deal.

The 1960s were considered to have "high" birth rates, seeing about 65,000 babies born every year.

These days, births were sitting around 60,000 a year - and staying there.

While mothers and fathers in their early 30s were having the most babies, men in their late 30s and early 40s having children had "doubled" since the 1980s, Hennessey said.

Researchers were noticing some trends across a number of countries when it came to a reduction in births, she said.

They included:

• Access to contraception. Women had more access to contraception, better education and more control over what was happening;
• More women were in education and paid work. That meant they needed to have more of a balance in life and often, were having children later in life, meaning there biological clock was ticking and they were more likely to have less children;
• Lower numbers of infant death. Hennessey said it used to be that women had more children in order to guarantee some reached adulthood - and that was no longer necessary.

A positive trend was the decrease in teen pregnancy in New Zealand.

Hennessey said the birth rate to teen mothers had "hugely dropped" and so had the abortion rate.

That showed there were fewer pregnancies, not just that teens were choosing a different path.

Data captured at birth helped Stats NZ learn more about the population, Hennessey said.

Useful information included the mother's age and location, as well as the sex and ethnicity of the baby.

It could also help Stats NZ project what would happen in the future and assume what fertility rates may be.