Observable variations in human sex ratio at birth

Area of Science:

• Reproductive Health
• Environmental Epidemiology
• Biostatistics

Background:

• The human sex ratio at birth (SRB) shows regional and temporal variations.
• Previous studies suggested environmental factors might influence SRB, but large-scale validation is needed.

Purpose of the Study:

• To validate known associations and test new hypotheses regarding factors affecting the human sex ratio at birth.
• To analyze large datasets for environmental and other influences on SRB.

Main Methods:

• Statistical analysis of two extensive datasets: US electronic medical records (EMRs) (~150 million individuals, 8 years) and Swedish National Patient Register (~9 million individuals, 30 years).
• Tested over 100 hypotheses related to seasonal changes, ambient temperature, air/water pollutants, and other environmental factors.

Main Results:

• No support found for seasonal variations or ambient temperature effects on SRB.
• Increased air and water pollutants (including industrial/agricultural activity proxies) were associated with lower SRBs.
• Certain environmental toxins were linked to higher SRBs, and new influencing factors were identified.

Conclusions:

• Environmental exposures, particularly pollutants, show modest but statistically significant associations with human sex ratio at birth.
• While not likely due to sex-specific selection, these findings are valuable for public health surveillance.
• Further empirical evidence is needed to corroborate these associations.