IS SEMEN QUALITY RELATED TO THE YEAR OF BIRTH AMONG DANISH INFERTILITY CLIENTS

Background. There is circumstantial evidence that human sperm count ma y have declined during past decades. The purpose of this study was to identify the association between semen quality and year of birth. Meth ods. The study comprised 8608 men consulting four Danish medical centr es from 1968 to 1992 because of infertility. Data on semen quality and urogenital disorders were obtained from medical records while lifesty le data were collected from a subset of the population by a postal que stionnaire (response 80%). Semen characteristics were analysed as a li near function of year of birth, centre, season and calendar year at ti me of semen examination, sexual abstinence and lifestyle factors. Effe cts 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 confounder s were adjusted for. Within the subset of men born 1950-1970 we reveal ed a decrease in the average sperm count by 1.9 mill/ml (95% confidenc e interval [CI]: 1.45, 2.27) per one advancing year of birth. This fin ding was consistent across centres even after adjustment for effects o f covariates. The proportion of morphologically normal sperm cells cha nged in parallel with the sperm count, while semen volume did not decl ine in any time periods. Conclusions. We found a birth cohort effect o n sperm count and morphology among Danish infertile men born after 195 0 but not in men born in the first part of the century. The findings a re compatible with an environmental impact during prenatal life but th e evidence is far from unequivocal.