Illicit drug use and health: analysis and projections of New York City birth outcomes using a Kalman filter model.

• Author: Mocan, H. Naci

1. Introduction

   As the use of illicit drugs persists as a major social problem facing urban America, the clinical evidence linking prenatal exposure to illicit drugs, in particular cocaine, and adverse birth outcomes has been mounting rapidly [8; 21; 38]. Newborns exposed prenatally to cocaine appear to be more likely to suffer intrauterine growth retardation, low birthweight and preterm delivery than infants unexposed. All three outcomes are strongly linked to infant mortality and excess morbidity [16; 22]. Furthermore, low birthweight infants who survive are more likely to experience serious developmental, health and learning problems later in life. The marginal costs of treating exposed as compared with unexposed infants has been estimated at between $9,313 and $13,225 in 1993 dollars for the initial hospitalization only [1, 29].

   The extent of the problem and its progression over time, however, are not well known. Estimates on the number of infants exposed to illicit substances in the United States range from 350,000 to 739,000 annually [9]. Determining the prevalence of illicit drug use in a free-living population is difficult and costly. The reporting biases inherent in surveys are substantial. More controlled studies based on urine toxocologies are limited in size as well as generalizability; and they provide little insight as to the changes over time [4].

   Indirect evidence suggests that the problem may be more widespread and more dynamic than anticipated. A univariate analysis of monthly time-series data from New York City birth certificates revealed a dramatic increase in the rate of low birthweight births among Blacks beginning in 1984. The rate of low birthweight rose from 10.6 percent in 1984 to 13.1 percent in 1988: an unprecedented rise which over four years reversed a 22-year decline [19]. The upturn in low birthweight among Blacks in New York City was coincident with anecdotal evidence on the introduction of crack cocaine to the city. Without time-series data on prenatal drug use, however, the nexus between low birthweight and cocaine was speculative.

   A more recent study based on New York City birth certificates addressed these shortcomings. Joyce, Racine and Mocan [18] used a pooled times-series, cross-sectional design to examine the link between illicit drug use and low birthweight. The study, based on annual data from 1980 to 1989 across health districts in New York City, reported that prenatal drug use was the most plausible explanation for the upturn in low birthweight among Blacks in the mid-1980s after controlling for prenatal care, smoking, and out-of-wedlock childbearing.

   Since early 1989, however, both prenatal drug use and low birthweight among Blacks have turned sharply downwards. The rate of low birth weight has fallen from a peak of 14.1 to approximately 12.2 percent in 1990. The observed rate of prenatal illicit drug use has fallen from 6.0 percent to 5.1 over the same time span. The turnaround in both low birth weight and prenatal drug use is certainly encouraging and it raises several questions. What is the most likely trajectory of prenatal drug use and its impact on the rate of low birthweight in the near term? What are the short-term opportunity costs of not pursuing an aggressive policy that reduces the prevalance of prenatal drug use to its pre-crack epidemic level? What would be the consequences and costs of an unexpected upturn in prenatal illicit drug use?

   To address these questions we fit a structural time-series model to race-specific monthly rates of low birthweight in New York City from 1963 through 1990. We first examine whether the upturn in 1984 is consistent with a structural shift in the long-term trend of low birthweight. We then project the rate of low birth weight that would have been observed had the upturn in 1984 not occurred. Finally, we add data on race-specific rates of prenatal illicit drug use, prenatal care and smoking that are available from 1978 to determine the relative contribution of each. With this model we provide answers to consequences and costs of various trajectories in illicit drug use and low birthweight.

2. Analytical Framework

   Economic models of infant health emphasize the distinction between the health production function and the input demand functions [5; 17; 32]. The former represents the technical relationship between the birth outcome and the health inputs, whereas the latter focuses on the factors which determine the use of the health inputs. To illustrate, consider the following system of equations.

   Birth Weight = [f.sub.1](Health Inputs, Substance Abuse) (1)

   Prenatal Care = [f.sub.2](Prices and Availability, Income) (2)

   Substance Abuse = [f.sub.3](Prices and Availability, Income) (3)

   In this paper, the birth weight is considered as a proxy for infant health, and the main health input is the consumption of prenatal care. Substance abuse stands for the consumption of deleterious substances such as tobacco and cocaine. The aim of the paper is the estimation of equation (1), the infant health production function. In particular, we seek to determine the contribution of illicit drug use to the rise in the rate of low birth weight. Note that substitution of equations (2) and (3) into equation (1) would yield a reduced form production function, where the rate of low birth weight becomes a function of input prices and income. Although clearly of interest, estimation of the reduced form is problematic, mainly because of unavailability of the price of illicit drugs, especially on a monthly basis.

3. Empirical Implementation

   The data include all singleton live births to Blacks and Whites residents of New York City between 1963 and 1990. Data on illicit drug use, however, are only available since 1978. New York City birth certificates are the only population based data source in the United States that has routinely reported information on prenatal illicit drug use for over a decade. The size of city, its racial diversity and the magnitude of the illicit drug problem make New York City a unique setting from which to address the time-series relationship between low birth weight and illicit drug use. The birth certificates contain information on prenatal substance abuse, which is based upon a combination of self-reports to physicians and positive toxicology screens applied at delivery. The potential consequences of this measurement are discussed in the results section.

   Individual birth certificates have been aggregated by month and race. Our measure of infant health is the race-specific rate of low birth weight births. Low birth weight is superior to infant mortality as a measure of health in time-series context, because there is less potential confounding in low birth weight due to technological change. The rapid decline in infant mortality in the United States over the past 20 years has been attributed to advances in management of newborn care. By contrast, the rate of low birth weight has shown only a slight improvement, which has been attributed to the increased utilization of appropriate prenatal care, better nutrition, and a declining proportion of births to adolescents [20; 27]. It is hypothesized that these favorable trends have been offset by the prenatal consumption of illicit drugs. Our measure of illicit drug use variable is the percentage of women whose pregnancies are complicated by the use of cocaine, heroin, methadone and barbiturates.(1) The production function estimated from 1978-1990 also includes the percentage of women who smoked during pregnancy, and the percentage of women who started consuming prenatal care during the first trimester. In preliminary analyses we experimented with other correlates of low birth weight such as percentage of births out-of-wedlock, the percentage of births to women with a high school education, and the percentage of first births; but these added little explanatory power to our model and were thus excluded from further specifications.

   Figure 1 presents the behavior of the Black and White rates of low birth weight. They span the years 1963 to 1990 and include 336 monthly observations. To highlight the underlying secular trends, both series are exponentially smoothed. The Black rate of low birth weight exhibits a downward trend between 1967 and 1984; it goes down from a monthly average of 14.66 in 1967 to 10.59 in 1984. Starting in 1985, the downward trend of 1967-84 is reversed, and the Black rate of low birth weight rises steadily until 1988, where the average reaches 12.95. The trend is reversed again in 1989, and the average value becomes 12.20 in 1990. Even though the White rate of low birth weight exhibits similar trends, the magnitudes of the changes are not as significant as those pertaining to Blacks. The monthly average rate of low birth weight for Whites is 7.23 in 1967, and 5.99 in 1984. It reaches 6.32 in 1988 and falls to 6.15 in 1990.

   The variables pertaining to prenatal care consumption, and the use of drugs and tobacco are displayed in Figures 2-4. Each variable spans the years 1978 to 1990, consisting of 156 observations. Figure 2 displays the percentage of pregnancies complicated by drug use for both races between 1978 and 1990. To make the graph comparable to the one presented in Figure 1, the years 1963-77 are left blank. The percentage of Black pregnancies complicated by drugs remains stationary between 1978 and 1983 (the mean is 2.35 in 1983-84), but we note an upswing starting with 1985, and a downturn in 1989 (the monthly average of 1988 is 6.67, and 4.87 in 1990). These dates coincide with the turning points of the Black rate of low birth weight series. For Whites, the corresponding averages are 2.27 in 1983-84, 3.08 in 1988, and 2.69 in 1990.

   Figure 3 shows the percentage of Black and White pregnancies complicated by tobacco use. Figure 4 reports percentage of pregnancies where the consumption of prenatal care started in the first trimester. The graphs...