Deborah Busch, 38, a' />

They're two proud and happy first-time parents with one very cute baby, but they are also part of a worldwide cultural shift.

Deborah Busch, 38, and Kerry Jacobs, 42, like increasing numbers of their counterparts worldwide, always planned to have a baby later in life and are relaxed about whether 4-day-old Zoe will have a brother or sister.

"A lot of my friends are having babies around this age, and she has lots of cousins to play with," Ms Busch said.

According to figures released by Statistics New Zealand in celebration of Mother's Day, the couple are typical of a population trend.

In the year to March, New Zealand women aged between 30 and 34 had the highest fertility rate, with the biggest drop in fertility among women aged 20 to 24.

New Zealand women are also averaging two children each - not enough for the population to replace itself and a trend worrying governments as a shrinking workforce will be faced with the healthcare and pension costs of retired baby-boomers.

Ms Busch, who is on maternity leave from her book-selling job, says baby Zoe was "planned like you wouldn't believe".

"We always had a feeling we would have children in our thirties so we could whack into the mortgage for a few years to make life easier," she says.

She and Kerry married five years ago. They have been together 17 years, have paid off their Sandringham home and finished house renovations a day before the baby was due.

Ms Busch said the hardest decision confronting her as an older mother-to-be was whether to have the amniocentesis test for Down's Syndrome.

The test has a 0.5 to 1 per cent chance of causing a miscarriage.

Older mums

* The median age for New Zealand women giving birth is now 30 years, compared with 25 years in 1975.
* New Zealand women are averaging two births each but to replace the population the rate needs to be 2.1.
* Natural population increase in the year to March accounted for 74 per cent of population growth, with immigration accounting for the rest.