Rotorua midwife Diane Travis said first-time mothers weren't the ones leaving children until their late 30s and early 40s.
Many of the women having babies at that age were already mothers having "unscheduled" subsequent babies.
And planned pregnancies after 35 could be rushed, she said.
"The 35-plus age group, once they start having babies, don't tend to wait big distances. They do have them fairly close together. Biologically they can't wait."
Kiwi mothers gave birth to 58,717 babies last year - a 4 per cent drop from 2012. Death numbers also fell slightly to 29,568 last year.
New Zealand's natural increase of live births minus deaths was the lowest since 2003.
University of Otago sociologist Bryndl Hohmann-Marriott said the rise in the number of older mothers was a continuing trend.
"New Zealand actually has one of the oldest ages of first birth in the developed world."
Once considered high-risk, births in the 35-39 age bracket were now much better managed and much more common.
Despite the slowing birth rate, the number of babies born here was still high compared with other developed nations, she said.
But a European trend towards fewer children could catch on here, Dr Hohmann-Marriott said.
"In a lot of those European countries like Italy, Spain and Germany where they're having so few children, people just don't want them anymore. For the first time there's a huge number of people who don't want any children at all.
"It's so difficult and so expensive to have [children]."
Slowing birth rates in Europe could largely be attributed to changing lifestyle priorities and the global financial crisis, she said, with uncertainty putting more people off having kids.
New Zealand's housing shortage could also be influencing people's decision not to have children, she said.
Statistics NZ analyst Anne Howard said fluctuations in birth rates could be a "challenge" for planners and schools.
