Six per cent of mothers use IVF or other artificial assistance to become pregnant, a study has revealed.
The "Growing Up in New Zealand" study of 7000 Auckland and Waikato babies found that 40 per cent of pregnancies were unplanned and 60 per cent planned.
The first set of data from the Auckland University study, which will follow the babies into adulthood, showed in 6 per cent of pregnancies, parents had had fertility treatment.
The type of reproductive assistance varied but the most common was IVF (in-vitro fertilisation).
Leading fertility specialists Dr Guy Gudex, of Repromed Auckland, and Dr Richard Fisher, of Fertility Associates, were surprised.
"We would normally quote a figure for New Zealand of 1.5 per cent of births being IVF," said Dr Gudex. "It's [3 per cent] for Australia."
These figures, from 2008, are based on formal reporting by fertility clinics and equate to about 900 IVF births a year in New Zealand.
"If you included any sort of fertility assistance you could probably double that 1.5 per cent," Dr Gudex said.
Dr Fisher estimated the proportion of New Zealand births arising from all fertility treatments could be 4.5 per cent - the higher, 6 per cent, finding in the Auckland University study might reflect a bias towards wealthier couples.
IVF can cost $11,000 and patients must find the money if they don't qualify for limited state funding.
The Growing Up paper said its sample was "representative of all current families having children in New Zealand" when measured by deprivation.
However the paper's graphs indicate the sample may contain slightly higher percentages of families from the "least-deprived" categories - an approximation for higher socio-economic status - than the New Zealand population has.
Dr Gudex said use of IVF and other fertility treatments was growing because of the increasing age at which women were trying to conceive and men's declining sperm counts.
The average age of mothers giving birth rose from 26 in 1979, to 30 last year, according to Statistics New Zealand; the average age of Growing Up mothers was 30.
But those in the study whose pregnancy was planned were older than the unplanned pregnancy mothers - 32 compared with 28.
Dr Fisher said surveys had shown that not wanting to interrupt careers was not the primary reason for women delaying child-bearing.
"One of the major reasons people don't have children earlier is they don't have a partner to have a baby with ... there's increasing use of donor insemination by single women.
"In the past, people met their partners early and tended to stay with them; they now tend to meet late and stay for a shorter period of time."