Many families can't afford to have more than one or two children.

New Zealand's new record low birthrate reflects how difficult and expensive it is to have a large family, says population expert Dr Natalie Jackson.

Jackson said women had fewer babies for many reasons, but she believed family economics played the biggest role.

"Women have paid off student loans, they are qualified, they want to work and it's very hard to juggle more than one or two kids and work."

Jackson said even the simple physical difficulty of trying to go to work and drop kids off at daycare and pick them up at night impacted on decisions on how many children to have.

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"A lot of people think they want to have three or four kids, but once they have had a couple, they realise it is very, very hard. It's a very big step to have the next one."

Stats NZ said the total fertility rate in 2017 was 1.81 births per woman, which is the country's lowest recorded level. In 2016, the average was 1.87.

The replacement level the country needs to maintain its population is 2.1 per woman, but the country's population continues to grow anyway because of immigration.

Jackson, who is a research associate at Waikato University's National Institute of Demographic and Economic Analysis, said increasingly having kids was just one option for couples, especially those who had started in their thirties or forties.

"You know, these days it's the house, or travel, or kids. It could be another kid or a house."

However, Jackson said New Zealand was only moving in the same direction as other Western countries.

"Across Europe, birthrates are 1.5 or 1.6. We are on the same sort of track."

She believed the new record low here was a combination of women having fewer children and the type of migrants New Zealand was taking.

"The population has grown because of the enormous number of people coming into the country. A very large proportion of the migrants are students and are Asian."

Students had other priorities than having children and Asians tended to have ultra low birthrates, like 1.4 births per woman.

The low birthrate had come from a drop in babies from women aged 15-29.

Jackson said those figures and the teen pregnancy percentage of 1.5 showed common negative stereotypes around teens and pregnancy were wrong.

"People still get all this moral panic about the teenage birthrate, but at 1.5 per cent you know that's actually very low. A large proportion of them are partnered too."

The same went for the stereotype of beneficiaries living on benefits having lots of kids.

"Figures go against that, too. What you find now is it's the people who can afford to have kids who are having the bigger families. If you look at it by social economic groups, the poorer families are not having the big numbers of births, it's just not true.

"There are a lot of prejudices when it comes to the birth rate."

Jackson said migration concealed a lot of what was happening around birthrates. She believed the true rate was probably about 1.9.

She said the population would continue to rise slowly and the birthrate could stabilise or even come up a bit depending on changes around migration.