Sex Ratio in Birth Outcomes: Measures and Data
What is the public health significance of sex ratio at birth?In the United States, sex ratio at birth was 1,049 male births per 1,000 female births in 2005, which is close to the expected rate of 1,050. According to the CDC National Vital Statistics Report, the U.S. population’s sex ratio fluctuated between a low of 1,046 and a high of 1,059 males per 1,000 female births during the 60 year period starting in 1942. The US sex ratio showed a downward trend following World War II, then an overall increase in the ratio of male to female births between 1959 – 1971, followed by another general decline in the last three decades.
Sex ratio at birth is used to monitor changes in the population structure, explain and project future birth and death rates, and monitor reproductive health, among other uses. This measure is important when examining infant morbidity rates, since male babies have a higher risk of low birth weight and infant mortality.
The environmental public health significance of the sex ratio at birth is still being explored. There are many potential factors affecting the sex ratio at birth (e.g., geographic differences, maternal age, birth order, stress, among others). Some researchers are examining the role of certain environmental toxics on declines in the sex ratio at birth, though it is difficult to tease out the causes of fluctuations in the sex ratio at birth. At present, it would be premature to point to changes in the sex ratio as a marker of a population’s exposure to environmental toxins. WTN maintains a list of recent publications that discuss the scientific evidence concerning the impact of environmental hazards on birth outcomes, including sex ratio at birth.
How is sex ratio at birth defined and tracked?Sex ratio at birth is the number of resident male live births divided by the number of resident female live births in a year, multiplied by 100 or 1,000. The rate is determined for a specific geography (e.g., nation) and specific time period (usually one year).
Data used to derive sex ratios originate with the birth certificate data. In Washington, the Department of Health (DOH) Center for Health Statistics tracks birth data through the Vital Registration System. The State Registrar sends these data to the CDC National Center for Health Statistics (NCHS). Data are standardized among all jurisdictions, and are generally available one year later than state data. NCHS routinely publishes statistics, including reports on sex ratio at birth, through the National Vital Statistics Report.
More Information on Specific Birth Outcomes