Marginal Changes in Semen Quality Over Three Decades (1996‒2022) Among 8320 Men in the Danish Young Men Study (DYMS): Cohort Profile and Trends

BACKGROUND: Since the 1990s, declining sperm counts and increasing testicular cancer rates have raised worldwide concern about male reproductive health, including in Denmark. This prompted a prospective population-based study with annual surveillance of semen quality. OBJECTIVE: This paper presents a detailed profile of the Danish Young Men Study (DYMS) cohort, including semen quality trends from 1996 to 2022 in the general population, interpreted alongside nationwide trends in testicular cancer. MATERIALS AND METHODS: The data include results of annual cross-sectional examinations (1996‒2022) of a total of 8320 men included from the general population (Copenhagen, Denmark) in relation to compulsory assessment for military service. Men provided semen, urine and blood samples, completed a questionnaire and underwent a physical examination. Temporal trends in semen parameters by examination year were assessed using linear regression. For sperm concentration, trends by birth cohort were further compared with nationwide trends in testicular cancer incidence. RESULTS: The median sperm concentration was 45 million/mL, total sperm count was 141 million, 61% progressively motile and 6.5% morphologically normal spermatozoa. Semen parameters remained relatively stable, but a modest improvement was observed in the most recent period (2017‒2022) compared to 1996‒2000; for example, sperm concentration increased from a median of 44 to 49 million/mL. Across the 26 years, 36% of participants had one or more semen parameters below WHO reference levels. A marginal increase in sperm concentration appeared from the earliest (1976‒1980) to latest birth cohorts (2001‒2004), coinciding with a plateau following the previous increase in testicular cancer incidence among Danish men. DISCUSSION AND CONCLUSION: Based on unique surveillance data spanning three decades, we showed persistently low semen quality with some temporal fluctuations among men included from the general population. This pattern aligns with the concomitant plateau in testicular cancer incidence. The modest improvement in semen quality observed in recent years may reflect favourable changes in environmental exposures or lifestyle factors, potentially related to the COVID-19 period. However, this finding should be interpreted cautiously, as it may represent random variation.