Key Points:

Young New Zealand women are having more babies sooner than their mothers did.

And a top Statistics NZ official believes the trend is a rebellion against the older generation.

The proportion of girls aged 15 to 19 having babies rose for the sixth year in a row, from a low of 2.6 per cent in 2002 to 3.1 per cent in 2007 and 3.3 per cent last year, Statistics NZ said yesterday.

The agency's principal demographer, Mansoor Khawaja, said young women appeared to be refusing to follow their mothers' decisions to have few children later in life.

"I reckon they just didn't agree with their mothers, which is not uncommon," he said.

"If you look at the previous generations, the mothers of the baby-boomers had roughly four children on average. The baby-boomers [born between 1945 and 1965] have ended up with less than two children each.

"There might have been a generation gap between the mother and the daughter. It's very interesting that every generation reverses the pattern of their mothers. They go back to their grandmothers."

Teenage mums are still only a small fraction of all teenagers. But the new figures confirm a trend towards higher birth rates at younger ages, which can no longer be dismissed as a temporary "blip".

There is still a long-term underlying trend towards women having babies later in life.

Birth rates for women in their 30s are on a steady upward trend which dates back almost 30 years.

But the birth rates for all age groups under 30 declined from the early 1960s until early this century. All are slightly up again since 2002.

The median age of mothers giving birth rose from 25 in the 1960s to 30 by 2002 and has stayed there since, dropping by a month or two in each of the past three years.

A 20-year-old mother at Auckland's Eden Campus teen parenting unit, Claire Trethowen, was an only child but already has a son aged 4 and hopes to have another two children eventually. "I do hope to have more because then they have other ones to play with."

But Emily Collins, the 18-year-old mother of a 2-year-old, does not plan to have any more children.

"I'm stuck with one and it was a total accident. I wasn't really thinking to rebel," she said.

Waikato University demography professor Ian Pool said he was not convinced that the slight rise in the birth rate would continue.

"We haven't got a massive trend, and we are really looking at the tea leaves trying to work out what is going on," he said.

Statistics NZ said birth rates had also risen in the past six years in Britain, France, Sweden, Denmark and Australia, where the total fertility rate - the number of children the average woman will have - has risen from 1.7 in 2001 to 1.9 in 2007.

New Zealand's rate has gone up from 2.0 in 2001 to 2.2 last year, the highest since 1990.

In Australia, many experts have linked the rising birth rate to the Howard Government's $5000 "baby bonus" paid to the parents of every new baby.

In New Zealand, paid parental leave and recent increases in family assistance may have had a similar effect.

Changing ethnic patterns also have an effect. Pacific women have an average of three babies each and Maori women 2.8, compared with 1.9 for European women and 1.5 for Asian women.

The buoyant economy may also have encouraged more couples to have children in the past six years.

Professor Pool and Mr Khawaja expect the recession to have a dampening effect in the next few years.