Semen quality in the 21^st century

Area of Science:

• Reproductive biology
• Environmental health
• Andrology

Background:

• Semen quality is a key factor in male fertility, but defining normal ranges remains challenging.
• Current World Health Organization (WHO) criteria for normal sperm concentration (15 × 10⁶/ml) may not reflect optimal fecundity, as 'time to pregnancy' studies suggest declines below 30-55 × 10⁶/ml.
• Recent studies indicate median sperm concentrations in young men (18-21 years) are 41-55 × 10⁶/ml, suggesting widespread suboptimal semen quality.

Purpose of the Study:

• To review the challenges in defining normal semen quality thresholds.
• To discuss trends in population semen quality and potential contributing factors.
• To highlight the importance of identifying environmental exposures impacting male reproductive health and fertility.

Main Methods:

• Literature review of 'time to pregnancy' studies and population-based semen quality assessments.
• Analysis of trends in sperm concentration over the past 15 years.
• Examination of potential associations between semen quality and factors like maternal smoking and chemical exposures.

Main Results:

• Fecundity declines when sperm concentration falls below 30-55 × 10⁶/ml, contrasting with the WHO's 15 × 10⁶/ml criterion.
• Median sperm concentrations in young men suggest suboptimal semen quality in a significant portion of the population.
• Sperm numbers are stable between ages 19-29, emphasizing developmental influences.

Conclusions:

• Suboptimal semen quality is prevalent in young men, necessitating a re-evaluation of normal ranges.
• Maternal smoking during pregnancy and chemical exposures causing endocrine disruption are potential contributors to poor semen quality and adverse trends.
• Identifying specific environmental exposures is crucial for developing effective prevention strategies for male reproductive health issues.