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Differences in Statistics about Births to Unmarried Women

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Today, the U.S. Census Bureau released a new report from the American Community Survey (ACS) looking at women aged 15 to 50 who gave birth in the last year and were unmarried at the time of the survey.

As of 2011, 62 percent of women age 20 to 24 who gave birth in the previous 12 months were unmarried. This compares with 17 percent among women age 35 to 39.

The ACS provides a variety of characteristics of mothers, providing detailed information about differences among groups of mothers. For example, the report released today highlights that the proportion of recent births to unmarried women varies widely by educational attainment and household income level.

The report shows us that more than half (57 percent) of women with less than a high school degree in 2011, who had given birth in the past year were unmarried, the highest percentage among the education groups. In contrast, only 9 percent of recent mothers with a bachelor’s degree or higher were unmarried.

In addition to the statistics available from the ACS, you can also find information from the National Vital Statistics System (NVSS), which tracks births to women living in the United States throughout the year. Overseen by the National Center for Health Statistics (NCHS), the NVSS provides administrative counts of births in the United States and basic characteristics of the mothers such as age, race and marital status. However, the detailed characteristics from the ACS allow us to provide a fuller profile of differences among groups of mothers, such as educational attainment, income and nativity.

We see two main differences between these two data sets. 

First, the NCHS’s vital statistics system records information on all births. On the other hand, the ACS is a survey, and while it is nationally representative, it does not have information on every birth that occurred in the United States. 

Second, the time frames covered by vital statistics and the ACS are quite different.  Birth records reported through the vital statistics system are collected at the time of the birth itself and reported for a one-year period. The ACS is a continuous survey that interviews respondents throughout a calendar year, asking them whether they had a birth in the 12 months prior to the interview. So, births reported in the 2011 ACS data could have occurred as early as January 2010 or as late as December 2011.

The difference in time frame affects other characteristics as well, including marital status and place of residence. ACS respondents report their marital status at the time of the interview, which may differ from their marital status at the time of the birth. It is possible that some of the respondents who indicate in the ACS that they are unmarried and had a birth in the last 12 months may have been married at the time of the birth, even though they were unmarried at the time of the survey. It is also possible that some of the respondents who indicated that they are married and had a birth in the last year were unmarried at the time of the birth but got married before the survey interview date.

Another source of differences between vital statistics counts and ACS estimates is that birth certificates are filed at the place where the birth occurred, while the ACS records the place the mother is living at the time of the survey, provided she has resided there more than two months.

Despite these differences, the ACS offers the important advantage of collecting a vast range of social demographic, and economic information about the women to whom these births occurred and the households in which they lived. The NVSS provides administrative counts on overall numbers of births.

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