title: Is semen quality related to the year of birth among Danish infertility clients?
abstract: BACKGROUND
  There is circumstantial evidence that human sperm count may have declined during past decades. The purpose of this study was to identify the association between semen quality and year of birth.
  
  
  METHODS
  The study comprised 8608 men consulting four Danish medical centres from 1968 to 1992 because of infertility. Data on semen quality and urogenital disorders were obtained from medical records while lifestyle data were collected from a subset of the population by a postal questionnaire (response 80%). Semen characteristics were analysed as a linear function of year of birth, centre, season and calendar year at time of semen examination, sexual abstinence and lifestyle factors. Effects of age were accounted for by restriction and stratified analysis.
  
  
  RESULTS
  The sperm count declined with increasing year of birth at two of the four centres, but this association disappeared when confounders were adjusted for. Within the subset of men born 1950-1970 we revealed a decrease in the average sperm count by 1.9 mill/ml (95% confidence interval [CI]: 1.45, 2.27) per one advancing year of birth. This finding was consistent across centres even after adjustment for effects of covariates. The proportion of morphologically normal sperm cells changed in parallel with the sperm count, while semen volume did not decline in any time periods.
  
  
  CONCLUSIONS
  We found a birth cohort effect on sperm count and morphology among Danish infertile men born after 1950 but not in men born in the first part of the century. The findings are compatible with an environmental impact during prenatal life but the evidence is far from unequivocal.
authors.affiliation: Steno Institute of Public Health (Departments of Occupational Medicine and Neurobiology), University of Aarhus, Denmark.
authors.forename: Y
authors.initials: Y
authors.lastname: Zheng
authors.forename: J P
authors.initials: JP
authors.lastname: Bonde
authors.forename: E
authors.initials: E
authors.lastname: Ernst
authors.forename: J T
authors.initials: JT
authors.lastname: Mortensen
authors.forename: J
authors.initials: J
authors.lastname: Egense
mesh_terms: D000328:Adult; D000368:Aged; D000375:Aging / Q000502:physiology*; D003718:Denmark / Q000453:epidemiology; D005260:Female; D005298:Fertility / Q000502:physiology; D006801:Humans; D007248:Infertility, Male / Q000453:epidemiology / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D012661:Semen / Q000502:physiology*; D013076:Sperm Count; D013094:Spermatozoa / Q000502:physiology