Selection bias in prenatal care use by Medicaid recipients.

OBJECTIVE: This study examines whether four types of selection bias in estimates of the effectiveness of prenatal care utilization for improving birthweight occur in a population of economically disadvantaged women.
METHODS: We categorized adequacy of prenatal care use using the Adequacy of Prenatal Care Utilization Index (APNCU) and the Revised-GINDEX for 142,381 Medicaid recipients who gave birth to a live, singleton infant in Washington State (1994-1998). Multinomial logistic regression was used to model categories of adequacy of prenatal care use as functions of variables chosen to indicate high- or low-risk status. A series of linear regression models were estimated to quantify the magnitude and direction of any bias in the effects of prenatal care on birthweight that could be attributed to accounting for each risk covariate. Results were examined for patterns of risk, prenatal care use, and estimation bias equated with the four selection processes.
RESULTS: We found modest evidence of adverse, favorable, confidence, and estrangement selection biases. The overriding effect, relative to low prenatal care use, was overestimation of the adequate care coefficient by 8.68 g with the APNCU, and underestimation by 3.36 g with the R-GINDEX because of competing confidence and estrangement selection biases. Relative to intensive use, the effect of adequate care on birthweight was underestimated (17.58 g with the APNCU; 13.34 g with the R-GINDEX) because of adverse selection bias and a small countervailing favorable selection process.
CONCLUSIONS: The underestimation of birthweight associated with prenatal care noted in prior studies appears to result from multiple selection processes working in different directions. Understanding selection processes can help the assessment of the contribution of prenatal care to birth outcomes and development of appropriate programs and policies.