do not do so. In 1984, for example, although some 7.5 million women, infants, and children were eligible, only slightly more than 3 million received WIC benefits (U.S. Department of Agriculture, 1987). It is generally believed that lack of knowledge about available benefits and administrative barriers to enrollment are in great measure to blame for lack of access to the program.
The primary source for data on prenatal care is the birth certificate. The data are reported by states annually to the National Center for Health Statistics (NCHS). Another important source of information about prenatal care is the 1988 National Maternal and Infant Health Survey (National Center for Health Statistics, 1991), which gathered data from mothers and their health care providers.
There are two aspects of prenatal care that are frequently measured: its initiation and frequency. When a woman first obtains prenatal care is important because care initiated early in a pregnancy has the best chance of preventing or treating medical conditions that could potentially harm the mother or fetus. Similarly, how often a women receives prenatal care is important, too, because periodic monitoring (with frequency determined by need) is essential for ensuring a good pregnancy outcome.
Because many insurance plans do not cover prenatal care and because Medicaid does not reimburse for these services at levels high enough to encourage all providers to participate, income is an important barrier to access. However, securing direct evidence of the link between income and access to care on a routine basis is difficult since income information is not reported on birth certificates. The Health Resources and Services Administration is testing the feasibility of combining the information provided on birth certificates with income data by zip code from the Census Bureau to estimate the income levels of women who use varying quantities of prenatal care services. Preliminary results from a pilot study in New York City indicate that living in lower-income neighborhoods is correlated with less use of prenatal care (Zeitel et al., 1991).
Several factors may affect the accuracy and usefulness of various measures of prenatal care. For example, none of the several measurement methodologies in widespread use defines in any precise way the components of a typical prenatal care visit (Institute of Medicine, 1988). In addition, several measurement methods rely for data collection on the memories of pregnant women or on their medical records, both of which can be faulty. Even the accuracy of birth certificates, used by the NCHS to generate most of the available information about prenatal care, has been called into question (National Center for Health Statistics, 1983; see also NCHS, 1980a).