Sex ratio: facts & figures

The ratio of male to female births

The expected sex ratio in the US is 1.05, or 105 boys born for every 100 girls. A sex ratio above 1 means there are more males than females, while a sex ratio below 1 means there are more females than males.

Sex ratio in Minnesota varies slightly over time

Sex ratio in MN and the US
Note: ratio of male to female births, among full-term, singleton births.

There are only small racial/ethnic differences

Sex ratio in MN by maternal race/ethnicity, 2009-2013
Note: ratio of male to female births, among full-term, singleton births. Racial categories White, Black, Asian/Pacific Islander, and American Indian are non-Hispanic. 

The sex ratio varies only slightly by mothers' race/ethnicity. 


Very small differences by maternal age

Sex ratio in MN by maternal age, 2009-2013
Note: ratio of male to female births.

In general, a low sex ratio at birth is associated with mothers' older age at childbirth.


Factors that influence sex ratio are not well understood

Environmental factors such as exposure to chemicals that disrupt the endocrine system also may result in fewer male births, lowering the sex ratio. Endocrine disrupting chemicals are a group of synthetic and natural chemicals that may alter or affect the endocrine system in animals and possibly humans.

Plastic additives like phthalates and bisphenol A (BPA) are two of a number of chemicals that have been implicated as endocrine disrupting chemicals. However, the environmental factors that may influence sex ratio are not well understood, and additional research is needed to understand their affects on human health.