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Massachusetts Department of Public Health seal Massachusetts Environmental Public Health Tracking

Sex Ratio at Birth

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The sex ratio at birth is the ratio of male to female births. In 2011, the sex ratio at birth (male to female) in the United States was 1.05. Slightly more males were born than females.

Did You Know?
The Northeast region has the lowest sex ratio in the U.S. Source: U.S. Census Bureau 2011

Numerous studies have reported changes in the ratio of males to females at birth with a reduction in male relative to female births in different countries throughout the world. Although the mechanism which determines the sex of the infant is not completely understood, some studies have suggested that environmental hazards, particularly endocrine disruptors, can affect how many males are born.

Other factors besides environmental exposures can affect the sex ratio. A reduced sex ratio at birth has been linked to older-aged parents and parental smoking and an increased sex ratio has been linked to premature births.

Data Considerations

When reviewing and interpreting sex-ratio at birth data, it is important to take into consideration the following:

  • Sex ratio does not consider the inability to become pregnant, which may potentially be caused by environmental exposures.
  • The data presented are based on the location of the residence at the time of birth. The place of residence or potential exposure during gestation or at the time of conception, when an exposure that may have affected the outcome could have occurred, may be different.

For additional information, please read the FAQ

Available Data on Sex Ratio

Use the Explore Maps & Tables link on this page to access the following measure for sex ratio in your community. The most current available data will be shown. Be sure to check the site periodically as new data are added each year. To protect privacy, no information is shown that could identify an individual.

  • Annual male to female sex ratio at birth (term, singleton live births only) by community, county, and statewide

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