Waikato University demographer Ian Pool said the blip was a faint "echo" of a high adolescent fertility rate around 1970. Children born then caused a first upward blip when they had children around 1990, and their children born around 1990 began having babies in the late 2000s.
The latest blip was also partly a "catch-up" effect as women who put off having babies in their 20s finally started having them in their 30s.
Now the blip is over, the country has reverted to a norm which has been slightly below replacement rate in most years since 1980.
Dr Pool said fertility rates were now lowest where women were better educated, had higher incomes and were more likely to be in paid work.
"In both Auckland and Wellington you have a central city/outer city divide," he said. "In Auckland it's Auckland Central and the North Shore versus higher rates in the west and much higher rates in South Auckland. In Wellington, it's Wellington City, the Hutt and Porirua in that order."
These patterns are partly ethnic. Fertility rates are highest for Pacific women (2.73 babies per woman) and Maori women (2.49), while European women equal the national average (1.92) and Asian women have far fewer babies (1.69).
But Dr Pool said Maori, as well as European women, were having fewer babies in the central cities.
Dr Susan Morton, who directs the Growing Up in NZ study of about 7000 children born in 2009-10, said women with one child who had not had any more children since the study started "tend to be older mums and better educated and living in more advantaged areas with higher incomes".
"There are certainly more women in higher education, and there is that old adage that a PhD is the best contraceptive," she said.
Dr Pool said another factor in declining fertility was the growing number of Asians, up from 2.3 per cent of the population in 1991 to 11.8 per cent in 2013. Fertility was still higher in Anglo-Saxon countries.
One's enough for Shelley
Shelley Bridgeman put off having a child until she was 38 - and then decided not to try again.
"I was quite focused on my career in my 20s and 30s," said the nzherald.co.nz columnist. "Lots of women are very sure they are going to have children. I wasn't like that."
Bridgeman, who has just turned 50, and her 51-year-old lawyer husband Kevin Jaffe started trying to have a child when she was 35. She had two miscarriages before giving birth to daughter Katie, who will be 12 next month.
"I remember thinking if we want another child I might have to go through that [miscarriage] again," she said.
The Auckland couple considered trying again when Katie was 2, but decided against it.
"We really thought at our age we were quite lucky to have one," she said. "If I'd had her at 30 it would have been different."
Bridgeman sees many advantages to a one-child family. Katie is a keen equestrian and the family can allow her all the time she wants to pursue it at weekends and after school.
Bridgeman said Katie was "very outgoing" and hadn't suffered by not having siblings.
