Asthma in Pregnancy: METHODS
The methods used have been described in part elsewhere but are summarized here. The data for this historical cohort study were obtained from an administrative database that contains linked birth certificate, infant death certificate, and maternal and newborn hospital discharge claims data for the delivery hospitalization for all singleton, live births to New Jersey residents in New Jersey hospitals during 1989-1993 (N=556,597). Ninety-five percent of births were successfully linked back to the birth certificate data. The mother’s and infant’s hospital discharge data contain information extracted from discharge summaries of their delivery hospitalizations.
Emergency room visits and hospitalization for asthma are not included in this database.
For the purpose of this study, mothers with medical claims with an International Classification of Diseases, Ninth Revision (ICD-9-CM) diagnosis code of 493 (asthma) in any of the diagnosis fields were considered to have asthma during pregnancy.
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Maternal race and ethnicity were obtained from the birth certificate data. These variables were used to categorize mothers into one of the following categories: non-Hispanic white, non-Hispanic African-American, Hispanic (regardless of race), and other (Asian, American Indian, other, and unknown categories). The mother’s level of education, marital status, age, parity, and smoking status during pregnancy also were derived from the birth certificate data.
The mother’s primary health insurance, available on the hospital Unified Billing Patient Summary at the time of delivery, was used to categorize mothers into one of the five insurance types: Medicaid, Medicaid HealthStart, self-pay, managed care, or (traditional) indemnity. New Jersey HealthStart is a comprehensive maternity services program, initiated under a Medicaid waiver in 1988, with prenatal care site certification. Pregnant women up to 185% of the federal poverty guidelines were eligible during the time period of this study. The range of services provided to women eligible for Medicaid changed based on the award of HealthStart certification. Case coordination, psychosocial, and health education components were offered to clients in addition to medical care.
Prenatal care was defined using two approaches: 1) the trimester of prenatal care initiation; and 2) the Adequacy of Prenatal Care Utilization (APCU) Index developed by Kotelchuck. The APCU index assesses the utilization of prenatal services by considering both the timing of initiation of prenatal care and the adequacy of service utilization after initiation, and characterizes utilization as inadequate, intermediate, adequate, and intensive or adequate plus care. The variables used to define prenatal care were derived from the birth certificate data. The first dimension of the
APCU Index assesses the adequacy of timing of initiation of prenatal care by the month prenatal care begins (months 1 and 2, months 3 and 4, months 5 and 6, and months 7-9). The second dimension of the index is the adequacy of received services. It attempts to evaluate the adequacy of the prenatal care visits received from the first prenatal visit until delivery. This is accomplished by measuring the ratio of the actual number of visits to the expected number of visits. The American College of Obstetricians and Gynecologists prenatal care visitation standards for uncomplicated pregnancies, after adjustment for the gestational age at initiation of care and for the gestational age at delivery, was used to estimate the expected number of visits. The APCU index is believed to offer a more accurate and comprehensive measure of prenatal care utilization, as compared to the trimester of prenatal care initiation. It should be noted that this index does not adjust for the intensity of visits (care) that is commonly found in high-risk pregnant women and does not assess the quality of prenatal care rendered.
For these analyses, the observational unit was the birth, not the claim. Maternal during pregnancy was the dependent variable. The independent variables of interest were race/ethnicity, insurance type, level of maternal education, the trimester of prenatal care initiation, and the APCU index.
The statistical models were constructed according to the conceptual framework of Williams et al. Multiple logistic regression was performed to examine the association between asthma during pregnancy and the race/ethnicity, insurance, maternal education, and prenatal care categories, after controlling for the potential confounding effects of maternal age, parity, and maternal smoking during pregnancy. First, separate analyses relating asthma to race/ethnicity were performed for each subcategory of health insurance, maternal education, and prenatal care initiation/utilization. White women were the reference group within each of these subcategories. For example, Medic-aid-enrolled white mothers were the reference group for the Medicaid race/ethnicity comparison.
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In the second stage of the analyses, within each of the three race/ethnicity categories, the independent association of maternal asthma with insurance type, maternal education, and prenatal care initiation/utilization was assessed before and after accounting for potential confounders. For these analyses, the reference groups were >16 years of completed education for level of maternal education category, indemnity for insurance category, first trimester for prenatal care initiation, and adequate care for the APCU index. For instance, African-American women with indemnity insurance were the reference group for insurance comparisons involving African Americans only.
In the third stage of the analyses, we evaluated the individual contributions of insurance type, level of maternal education, and prenatal care initiation/ adequacy, using a sequential multivariable model. We observed how the estimates of the association between race and asthma changed as we sequentially included insurance type, maternal education, and trimester of prenatal care initiation. Other potential confounders, including age, parity, and smoking history, also were included in this model. This analysis was done with and without stratification for prenatal care adequacy and initiation.