People wanting to start a family are being warned to plan for their last baby rather than their first, or risk having heartbreaking consequences.

The country's fertility rates are falling, with one in four New Zealanders having problems falling pregnant.

Women are most fertile around the age of 30, but the chances of conceiving naturally dramatically drop once they turn 35.

For men, that age is 50.

Fertility New Zealand spokesperson Juanita Copeland told Rachel Smalley 60 per cent of women underestimate their fertility window.

"What we're saying is a lot of Kiwis have a real mental and emotional picture of what they want their family to look like. And if you have that target in mind, you need to think about the end game first."

LISTEN TO JUANITA COPELAND TALK TO RACHEL SMALLEY ABOVE