A new study suggests the quality of sperm in New Zealand men has more than halved in two decades.
The data has been presented to international fertility researchers in Brisbane and is due to be published in the New Zealand Medical Journal.
It is based on sperm quality figures from volunteer donors, which show sperm volume dropped from 110 million per millilitre to 50 million per millilitre from 1987 to last year.
While dramatic, John Peek of Fertility Associates says the fall represents a drop from "very good" to "good" sperm quality.
Dr Peek says theories for the fall include environmental factors and effects on male babies in the womb.
However, he says the level seems to have recently stabilised and is adequate to maintain fertility.