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Last Edited: 9/30/2010

Sex Ratio at Birth: Measurement and Limitations

How are sex ratios calculated?

Epidemiologists most frequently calculate sex ratio at birth by taking the number of male newborns recorded on birth certificates and dividing by the number of female newborns.  In large populations, this number is most often in the neighborhood of 1.05, since boys are born slightly more often than girls.

Researchers utilizing these data may wish to know that we used a common modification of Wilson’s approach for the calculation of these intervals.  This approach is designed for use with binomial variables, so we begin with estimates cast as the proportion of males and transform the resulting confidence limits into odds.


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Where do these data come from?

In the United States, states are responsible for issuing birth certificates and recording and maintaining the data included in them.  Birth certificates are considered one type of vital record (others include deaths, fetal deaths, and marriage).  In California, the Office of Health Information and Research (OHIR) is responsible for stewardship and distribution of vital statistics data and provides written reports and data tables analyzing these data. Since several of the important functions of the CEHTP include the analysis and processing of these records, we maintain our own databases consisting of records produced by OHIR and subject  them to further processing, most notably regarding address and other geographic information fields.


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For the majority of births, the sex recorded on birth certificates is believed to be accurate.  Newborns with rare medical conditions may have genitalia that do not match the sex one would expect from their chromosomal make-up; it is unknown the degree to which this affects the recording of sex in birth certificate records.  It is also difficult to assess the relationships between sex ratios at birth of populations with those at other time-points, such as conception or during adulthood.

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