Prime Minister Jacinda Ardern has kicked the trend of struggling to conceive aged 37.

When announcing her pregnancy with partner Clarke Gayford she mentioned they were "lucky" to have conceived naturally.

"Clarke and I have always been clear we wanted to be parents but had been told we would need help for that to happen. That's made this news a fantastic surprise."

Infertility - defined as not conceiving after 12 months of trying - affects one in four New Zealand couples, support organisation Fertility NZ figures show.

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Five years ago it was one in six.

The main factor in increasing infertility is couples are choosing to have children later in life, Fertility Associates' Auckland medical director Dr Simon Kelly explained.

"Women are expected to have a career now and a family. It's just the way it is. There are no signs this is going to change."

Female fertility starts to decline at age 32, from age 35 the fertility decline speeds up and by 40 fertility has fallen by half.

Kelly urged couples to plan for their last child and not their first. For a 90 per cent success rate to have three children naturally, a woman needs to start trying at 23, the Oxford journal Human Reproduction found.

If they want two children, the woman should start trying at 27, and if they want one for a 90 per cent success rate, they should try no later than 32.

"If someone has a plan about what their ideal family size is, then take into account when you start trying," Kelly said.

"Say if someone starts at 35 with a view that they want three children. If they have one at 35, by the time they've had that baby and are thinking about number two they could potentially be 37 - the chance of conceiving may be reduced and you might need treatment.

"If they are successful, by the time they're ready for number three they could easily be in their 40s."

Infertility rates are at an all-time high. A quarter of New Zealanders struggle to conceive.

There are close to 6000 Assisted Reproductive Technology (ART) treatment cycles each year and about a quarter are successful.

A couple where the woman is aged 30 has about a 47 per cent chance of a baby from one IVF cycle. If the woman is aged 40, this drops to 23 per cent and less than 10 per cent for women aged over 42, Fertility NZ reported.

If a couple wanted a large family Kelly might suggest doing a round of IVF earlier as you can freeze embryos.

"It has huge benefit," he told the Herald.

"Someone at 36 could freeze two or three embryos. Then if they choose to have another baby a few years later they have a 36-year-old's chance of falling pregnant, not a 40-year-old's."