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Reproductive and Birth Outcomes

Sex Ratio and the Environment

The sex ratio at birth means the number of babies born male compared to the number of babies born female. The expected sex ratio at birth (male to female) is 105 boys born for every 100 girls. Studies in the United States, Canada, Denmark, and the Netherlands have reported changes in the ratio of males to females at birth. Some scientists have suggested that environmental hazards can affect how many males are born. Parents and the fetus can be exposed to different hazards referred to as endocrine disruptors. Fewer males are conceived when exposure to endocrine disruptors causes a decrease in testosterone.

Diethylstilbestrol (DES), a synthetic estrogen widely prescribed to pregnant women during the mid-1900s, is a strong endocrine disruptor. Previous studies have suggested an association between endocrine-disrupting compounds and the secondary sex ratio ( the sex ratio of the grandchildren of the exposed women).

Exposure and Risk

Decreases in male births may also be related to the following:

  • Parental smoking
  • Gestational length
  • Parental age
  • Birth order

Reproductive practices and social practices about sex preferences—males over females, for example, can affect sex ratio.

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Because states have complete vital statistics data on the sex of babies born, changes over time in the sex ratio can be measured as the ratio of male to female births. Several studies show that declines in the sex ratio of males to females at birth maybe associated with occupational exposure, or exposure to air pollution. Thus, prevention should focus on reducing such exposures.

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