InrodudionVital statistics data is a readily available source of information on prenatal care usage in the United States. How to examine vital statistics and other types of data for the effect ofprenatal care on pregnancy outcome has generated a number of different methodologies.1-4 One of the difficulties encountered in using such data has been the problem of overcoming the preterm bias effect. The preterm bias effect is associated with observing better outcomes in women who receive prenatal care than in women who receive no care. This artifact occurs because women receiving no care may deliver prematurely and may not have an opportunity to enroll in prenatal care.5 By conditioning the effect of the age of entry into prenatal care on the gestational age of delivery, however, the preterm bias effect can be avoided. Nonetheless, other problems encountered in vital statistics data may invalidate the results of the conditional approach.In this study, we demonstrate the effect that the possible misclassification of gestational age in vital statistics data has on the interpretation of the conditional approach and discuss the problematic nature of the evaluation of prenatal care.Metods justed analyses and 374 244 (96.7%) were used for the adjusted analyses.The association of the gestational age of entry into prenatal care with pregnancy outcome was examined relative to the gestational age of delivery. Outcomes (stillbirths or infant deaths) are presented as death rates (percentage dead). We calculated odds ratios for stillbirths or infant deaths by logistic regression models adjusted for race (Black vs non-Black), multiple births (multiple vs single), (0 parity and parity greater than 3 compared with parity 1 through 3), maternal age (less than 18 and greater than 34 compared with 18 through 34 years), and matemal education (less than 12 compared with greater than 12 years). Mantel-Haenszel tests for linear trends were carried out for mortality rates over the gestational ages of entry into prenatal care.6 Furthermore, we adjusted our results by declaring newborns that exceeded the 90th percentile birth-weight standards for gestational age reported by Arbuckle7 as misclassified.All computations were carried out using SAS@88 ResultsBy conditional analysis, the risk (percentage of mortality) of stillbirth or infant death for those women who delivered at 25 weeks to 28 weeks ofgestation and who entered prenatal care between 1 and 4 weeks was greater than the risk of stillThe data for this analysis were obtained from linked birth and death certificates from Missouri for 1980 through 1984. The variables used in the analysis were the gestational age at which prenatal care was initiated (self-report and limited to the first 36 weeks of pregnancy), the gestational age at delivery (based on last menstrual period), the outcome (stillbirth, infant death, or alive at 1 year), race (Black or non-Black), birth number (multiple vs single), parity, maternal age, and maternal education. We used only those records that involved gestatio...
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