Fertility experts are advising people not to delay having children for too long, saying biology is designed for reproduction until about the age of 35.
The increasing tendency to delay first-time parenthood and the problems this can pose has been a theme of a fertility conference in Auckland.
Experts say men and women are most fertile at age 19 or 20.
ProfessorJohn Aitken from Australian said that beyond 35, both sexes can face problems and sperm damage in men may pass on conditions such as schizophrenia, epilepsy and autism.
Mary Birdsall, of Fertility Associates, said New Zealand women commonly delay having children until about 30, but can then find it hard to conceive.
Dr Birdsall said one in six couples have infertility problems, which are shared equally between men and women.