Conflicting evidence about the extent to which men's semen quality declines with age - likely lowering their fertility - is being cleared up by a new review of data from 90 previous studies from around the world.
Researchers from the new University of Otago found consistent age-related declines in semen volume and sperm performance and increases in malformed and DNA-damaged sperm. Semen quality is regarded as a proxy for how fertile a male is.
Lead author Dr Sheri Johnson said understanding how age affected fertility was becoming increasingly important as couples delayed childbearing toward later stages of their lives.
"While female age is well known to have negative effects on fertility, reproductive success and the health of offspring, the influence of male age on a couple's fertility has been largely neglected.
"The effects of declining semen traits with increasing male age have largely been ignored due to inconsistencies in the literature, but our work now suggests that male age affects a variety of traits.
