People delaying having a family might find it more difficult than they think to conceive.
Researchers at the University of Otago say the proportion of adults with fertility problems by their late-thirties is higher than expected.
Infertility was defined as trying to conceive for at least twelve months, or seeking help to become pregnant.
They say of their study group, 22-percent of men and 26-percent of women experienced problems by the time they were 38.
Associate professor of epidemiology at the Dunedin School of Medicine Nigel Dickson says that is higher than in recent comparable studies around the world.
"That was a little bit higher than we'd expected. This was only by the age of 38, and it is likely that because infertility problems increase with increasing age, overall it'll probably be a greater proportion that would have had problems by the time they're 45."
More than 900 people took part in the research as part of the Dunedin Multidisciplinary Health and Development Study, which follows the health of a thousand adults throughout their lives.