Journal Issue: Low Birth Weight Volume 5 Number 1 Spring 1995
The Impact of Prenatal Care on Low Birth Weight
Although the general notion that prenatal care is of value to both mother and child became widely accepted during the twentieth century, the empirical evidence supporting the association between prenatal care and reduced rates of low birth weight emerged slowly and has been equivocal. The slow pace of research in this area reflects, in part, the complexity and variability of this intervention and the difficulties inherent in measuring the adequacy of its use and its content. Prenatal care has many aspects including, at a minimum, when it starts, the number and spacing of the visits, the content of each visit, the type of provider, the provider setting (for example, hospital, clinic, or home), the assessment of risk status, the schedule of medical screening tests, and the use of specific medical, educational, nutritional, and social support intervention services. The history of efforts to assess the impact of prenatal care documents the dogged determination of several generations of investigators to define and evaluate a complex and changing intervention with numerous components while being generally limited to the use of observational studies.
The collective evidence suggests that adequate prenatal care is associated with reduced rates of low birth weight but mainly among more mature full-term infants. Unfortunately, prenatal care has consistently not been shown to prevent fetal growth retardation among less mature preterm infants or to prevent preterm birth.7 The relationship between prenatal care and very low birth weight (less than 1,500 grams, or 3 pounds, 5 ounces) and very preterm delivery (at less than 33 weeks) is also uncertain. Programs established to prevent the onset of preterm labor and to facilitate its early identification and treatment also did not clearly reduce rates of preterm birth or low birth weight.8-10 These generally negative results underscore the lack of well-defined programmatic approaches currently available to prenatal care providers to prevent preterm birth.8,11
The results of randomized controlled trials that have attempted to evaluate differences in birth outcomes between standard prenatal care and more comprehensive medical, nutritional, educational, and psychosocial support service configurations have also been generally negative, although some have shown benefits for specific populations.10,12-15 A comprehensive program of prenatal nurse home visiting had no overall effect on birth weight or gestational age, but improvements in birth weight were noted for specific subgroups of women, for example, young adolescents.12
A prenatal intervention provided by nurse-midwives which included patient education, social support, and nutritional and substance-use counseling also resulted in no differences in low birth weight from programs in which obstetricians provided only usual care.10 In other studies, the addition of social support or enhanced prenatal care services also failed to reduce rates of low birth weight.13-16
Although the combined evidence from nonrandomized studies suggests a relationship between some prenatal care services and mean or low birth weight, the many threats to validity of inference inherent in these nonrandomized studies need appraisal.17-30 Nonrandomized observational studies do not control for the self-selection bias of women who may choose to use comprehensive rather than standard services. Further, if participation in a comprehensive prenatal care program is determined by the receipt of a minimum number of visits, gestational age must be considered as a potential confounder in the selection of a comparison group.
There is little definitive information on the extent to which specific individual components or some combination of components of standard or comprehensive prenatal care—for example, social support, home visits, education, and the like—may be effective.
A recent study, examining the relationship between the receipt of a recommended array of initial medical procedures and health promotional advice during prenatal care and low birth weight reported a positive benefit for health advice but no associated impact for the initial medical procedures.31 Variations by provider in the content of prenatal care have been noted, and there is mounting evidence that these variations may exist among population subgroups in the content of prenatal care they received.32-36
As many of these studies relied on mothers' memory of the advice and care that they received, the possibility of recall bias must be considered in the interpretation of these results.