Spatiotemporal trends in human semen quality.

Area of Science:

• Reproductive biology
• Environmental health

Background:

• Human semen quality has been studied for decades, with reported spatial and temporal trends.
• Geographical variations in sperm production are evident, even within short distances.
• Beliefs about declining sperm production over time are common, but scientific evidence is inconsistent.

Purpose of the Study:

• To evaluate the evidence for spatial and temporal trends in human semen quality.
• To critically assess the reliability of existing studies on semen quality trends.
• To identify limitations in current research and suggest future directions.

Main Methods:

• Review of various study designs, including standardized-methodology, cross-sectional, and retrospective multicentre studies.
• Analysis of data on sperm production, motility, and morphology.
• Assessment of study limitations, including population homogeneity, methodology, and data interpretation.

Main Results:

• Standardized studies show clear geographical differences in sperm production.
• Evidence for a temporal decline in human sperm quality is inconsistent and often based on questionable study designs.
• Studies on sperm motility and morphology are limited by assessment variability.
• Available data do not support a worldwide decline in semen quality, but trends exist in specific areas.

Conclusions:

• Human semen quality exhibits significant geographical variations.
• A global or Western-world decline in semen quality is not conclusively demonstrated.
• Prospective studies incorporating male exposome assessment are recommended to clarify trends and contrasts.