Journal Issue: Low Birth Weight Volume 5 Number 1 Spring 1995
Problems in Defining the Relationship Between Prenatal Care and Low Birth Weight
Defining an accurate way to measure the adequacy of prenatal care has been a major challenge. Over the years, it has been defined by the number of visits, the stage of pregnancy at which care was initiated, the source of care (private versus clinic, public, or ward), the spacing of visits, and/or the content of medical care and auxiliary services (for example, blood and urine tests, health education, and dental care).37-41 In 1962, Shwartz suggested that the association between low birth weight and the trimester in which care began or the number of prenatal care visits may well be confounded by gestational age.42 Mothers who delayed initiation of care until the third trimester are likely to have a declining risk of low birth weight as they are approaching full term when care begins. Conversely, increased birth weight may be expected to coincide with more visits because both birth weight and the total number of scheduled visits increase with gestational age. The importance of adjusting for the effects of gestational age when relating prenatal care use to birth weight has now been recognized by many investigators.7,42-45
The cumulative results of studies of adequate prenatal care utilization indices have had a dramatic influence on establishing a widespread confidence in the ability of early and frequent use of prenatal care to reduce the risk of low birth weight.45-47 Nevertheless, limitations to these studies have been recognized. Self-selection bias is a major concern in the interpretation of research in this area.5,43,46,47 Health-conscious women may be more likely to initiate prenatal care early and to maintain a regular schedule of visits. These same women may also demonstrate other health-care-seeking and health-promoting behaviors, including planning their pregnancies, obtaining preconceptional care, maintaining a proper diet, and abstaining from the use of tobacco, alcohol, and illicit substances. They may influence the content of their care through their selection of a prenatal care provider and through their requests for and adherence to provider advice on positive pregnancy-related behaviors. These health-promoting behaviors may contribute to reducing the risk of a low birth weight delivery. Hence, adequacy of prenatal care use could be an indicator of myriad health-enhancing maternal attitudes and behaviors as well as a measure of the prenatal care received. In the absence of findings from randomized controlled trials, this unaddressed self-selection bias leaves concerns as to whether simply improving the adequacy of prenatal care use will result in improvements in low birth weight.
There are other limitations to studies of prenatal care use. It should be noted that prenatal care utilization indices, which combine the timing of initiation and the number of visits, are unable to assess these components of the index separately or to address issues related to the content of the services received. (The two-part adequacy of prenatal care utilization index proposed by Kotelchuck, which separately assesses initiation and adequacy of visits, is a notable exception.48) Finally, lack of clarity still exists regarding some of the mechanisms or biological pathways by which prenatal care services influence low birth weight and its specific components of preterm and small-for-gestational-age birth. In more recent studies, researchers have found that the major impact of any prenatal care use on birth weight is limited to more gestationally mature infants.49,50
To illustrate the general findings of studies using adequacy of prenatal care indices, Table 1 presents birth weight characteristics by adequacy of prenatal care utilization groups for 1986 single live births to U.S.-resident mothers.51 Prenatal care utilization categories were defined using a modified version of the Kessner index.45,46 The majority of mothers (56%) demonstrated adequate utilization, and less than 7% reported inadequate utilization or no care. Among mothers of low birth weight infants, 42% adequately utilized prenatal care, while 13.5% exhibited inadequate or no prenatal care use.
The second section of the table provides low birth weight percentages for each category of prenatal care use. The moderately low (1,500 to 2,499 grams, or between 3 pounds, 5 ounces and 5 pounds, 8 ounces) birth weight percentages progressively increased with less than adequate prenatal care utilization (3.75% for adequate to 7.85% for inadequate). A similar progressive relationship between adequacy of prenatal care and very low birth weight (less than 1,500 grams) was not in evidence, however. It should be emphasized that the very low birth weight category is appreciably comprised of very preterm infants, a condition for which there is less evidence of a relationship with prenatal care use. While these data appear to demonstrate an association between adequacy of care and moderately low birth weight, they are fraught with most of the problems noted above and, thus, cannot be viewed as definitive.