Reproductive Health Indicator:  Sex ratio

Measure: Male to female sex ratio at birth

The sex ratio at birth is the ratio of male to female births. Many studies have found a reduction in male relative to female births in different countries throughout the world. When the number of male births is the same as the number of female births, the sex ratio is equal to 1.0. The baseline sex ratio is usually slightly above 1.0, typically close to 1.05. In 2006 the ratio in Oregon was 1.05. The current ratio in the United States is also 1.05.

Although the mechanism which determines the sex of the infant is not completely understood, some studies have suggested that environmental hazards, particularly exposure to endocrine disruptors, can affect how many males are born.  An endocrine disruptor is a synthetic chemical that either mimics or blocks hormones and disrupts the body's normal functions. Fewer males are conceived when exposure to endocrine disruptors results in a decrease in testosterone. Known human endocrine disruptors include dioxin, polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethane (DDT). Suspected human endocrine disruptors, based on limited animal studies, include plasticizers and various pesticides. Decreases in male births have also been shown to be inversely related to parental smoking, gestation length, parental age, and birth order. 

If a consistent decrease in the sex ratio occurs, tracking the change could help identify environmental hazards that can disrupt the endocrine system or some other physiological system related directly or indirectly to the expression of sex at birth.

The table, graph and map below show the sex ratio for 2000-2006 by county.  Counties with more than 10 births of each sex showed a broad range from a 2004 low of 0.61 in Wallowa and Grant counties to a 2002 high of 2.10 in Lake County. However, no temporal trends were identified. The low and high ratios for all years (0.90 and 1.26 in Wallowa and Harney counties, respectively) were in counties with few births and do not suggest meaningful differences in the sex ratio. 

NOTE: The sex ratio measure is based on term singleton births only. Term births are those for which gestation is equal to or greater than 37 weeks.

Table 1: Male/female sex ratio at birth (term singleton births only), by county and year.

Graph 1: Male/female sex ratio at birth by county, summarized for 2000 – 2006. 

Map 1: Male/female sex ratio of offspring at birth, shown in three statistical categories: higher than, similar to, or lower than the Oregon ratio.

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Table 1: Male/female sex ratio at birth (term singleton births only), by county and year.

Rates based on less than 10 births are considered unstable and marked with an asterisk (*).

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Graph 1: Male/female sex ratio at birth by county, summarized for 2000 – 2006.

Rates are shown with their 95% Confidence Intervals (CI). There are no significant differences among counties, as all county CIs overlap and include the Oregon ratio of 1.04 (indicated by the dotted vertical line).

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Map 1: Male/female sex ratio of offspring at birth, shown in three statistical categories: higher than, similar to, or lower than the Oregon ratio.

Data are summarized for 2000 to 2006. Statistical significance was determined by comparing the 95% confidence interval of the county rate with the State rate. All county ratios are considered statistically similar to the Oregon ratio of 1.04.

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