Maternal depression and the production of infant health.

• Author: Conway, Karen Smith

1. Introduction

   Understanding the correlates of infant health has become an issue of great importance since we have learned that it has tremendous implications for childhood and adult health and well-being. Low birth weight is a key indicator of infant health, as low birth weight babies have much higher mortality rates. In addition, caring for these low birth weight babies is substantially more costly (Currie and Gruber 1996). Perhaps even more compelling is the mounting evidence that the negative effects of being born at a low birth weight persist well into adulthood and can sometimes take many years to appear. (1) Reducing the incidence of low birth weight, possibly through improved prenatal care, is therefore a worthwhile policy goal, but what is the most effective way to do it?

   The recent, dramatic expansion in the Medicaid eligibility of pregnant women has been fueled, at least in part, by the belief that it would increase prenatal care and ultimately improve infant health. However, expanded eligibility does not necessarily lead to increased participation, as the take-up rate for Medicaid is tar below 100% (Gruber 1997). Furthermore, if individuals drop their private insurance, which is presumably of superior quality, on receipt of Medicaid, then expanding Medicaid may not improve infant health. (2) Finally, even if expanding Medicaid coverage leads to increased prenatal care, those increases may not substantially improve infant health if prenatal care is not effective.

   We suggest that a potentially important element has been left out of the policy discussion: the mother's mental health. Epidemiological research has shown that infants of depressed mothers show signs of poorer health (Abrams et al. 1995; Field 1995, 1998; Dawson et al. 1997a, 1997b; Jones et al. 1997; Locke et al. 1997) and are more likely to be preterm (Orr and Miller 1995; Orr, James, and Prince 2002). Maternal depression is not a rare event: The lifetime risk for depression for women is estimated at 10-25% and peaks during their childbearing years (Wisner et at. 1999; Desai and Jann 2000). Low-income women are even more susceptible to depression, and there is evidence that those on welfare suffer greater depression than other low-income women (e.g., Lennon, Blome, and English 2001). Yet current economic research completely overlooks the role that maternal depression plays in infant health.

   In studies of welfare reform, there is a growing recognition of the role depression plays in preventing women from getting off welfare and finding and keeping employment. For instance, Lennon, Blome, and English (2001) provide an extensive survey of the prevalence of depression among low-income women and the consequences it has for them. They also discuss and evaluate different kinds of welfare and unemployment programs that incorporate treating mental illness. Our results suggest that perhaps the Medicaid program and, more generally, the health care providers who treat pregnant women also need to recognize the important role that maternal depression plays in infant health.

   We begin with a standard infant health production model in which "depression" may affect infant health directly as part of the technology of production and also as a factor that influences the choice of inputs (e.g., prenatal care). Using the National Maternal and Infant Health Survey (NMIHS), we estimate birth weight and prenatal care equations that incorporate measures of maternal depressive symptoms. (3) Because past researchers have found significant differences between whites and blacks, we stratify by race. The NMIHS does not contain the ideal measure of maternal depression--a diagnosis of depression during the pregnancy--and so we construct several measures to verify the validity of our results. One byproduct of constructing these measures is an investigation into factors associated with maternal depressive symptoms. Across all measures and samples, we consistently find that maternal depressive symptoms have a negative direct effect on birth weight and that these symptoms may operate through other channels, such as reduced prenatal care and increased unhealthy behaviors.

2. Production of Infant Health

   We extend the infant health production model of Rosenzweig and Schultz (1982, 1983) by adding "depression" as both a factor that affects the technology of production and a taste variable. The mother's utility function is defined as

   (1) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] utility function,

   where H reflects her infant's health, D is her mental state or "depression," and X is a composite commodity. The mother then maximizes this utility function by choosing prenatal care, M, subject to the infant health production function and her budget constraint,

   (2) H = H(M,D,Z) infant health production

   (3) PM + X = I budget constraint.

   Referring to Equation 2, prenatal care, M, is expected to enhance infant health, H, whereas depression, D, is expected to adversely affect infant health. In addition, other variables (Z) such as the mother's education, age, height (as suggested by Warner 1995, 1998), and past medical history, as well as other maternal behaviors, are expected to directly affect infant health. Equation 3 represents the budget constraint, where P is the out-of-pocket price per unit of prenatal care, I is income, and the price of the composite good is normalized to 1.0.

   The woman then chooses the prenatal care, [M.sup.*], that maximizes her utility subject to Equations 2 and 3, which yields the two-equation model typically estimated--the desired prenatal care demand equation,

   (4) [M.sup.*] = M(D,Z,P,I),

   and the desired infant health equation,

   (5) [H.sup.*] = H([M.sup.*], D, Z).

   This is the typical framework employed by recent studies of infant health and prenatal care (e.g., Grossman and Joyce 1990: Warner 1998; Currie and Grogger 2000). It demonstrates how variables such as age and education can have both a direct and indirect (through prenatal care) effect on infant health. We introduce maternal depression as another such factor that can have dual influences--both on the decision to get prenatal care as well as having a direct effect on the production of infant health. This view is consistent with the epidemiological literature, such as Orr and Miller (1995), which also has recognized these dual avenues. In particular, Orr and Miller (1995, p. 169) in their review of maternal depressive symptoms and pregnancy outcomes note the "two primary mechanisms by which depressive symptoms might influence birthweight": by possibly leading to more harmful behaviors (including delayed prenatal care) and by "a more direct association." as "depressed mood has been increasingly linked to biochemical/hormonal alterations in the body."

   One might argue that depression may be jointly produced with infant health (health inputs affect both). However, in a recent study, Williams et al. (1999, p. 64) find that among family physicians, general internists, and obstetricians-gynecologists (OB-GYNs), OB-GYNs stand out as having higher physician barriers to treating depression (e.g., having low confidence in or incomplete knowledge of treatment). It therefore seems unlikely that the woman is seeking prenatal care to treat her depression. We also hesitate to make the assumption implied by such an argument that maternal depression is a choice variable and under the mother's control. (4) However, maternal depression may be endogenous to birth weight in that the mother's unobserved health endowment may affect both. With this caveat in mind, our estimated effects of observed maternal depressive symptoms should be viewed as measuring their value as signals of maternal health and behavioral problems that could lead to a poor birth outcome.

   Despite our reservations about treating depression as a choice variable, we modify this framework to allow for such a possibility in our empirical analysis. In essence, we specify a reduced-form maternal "depression" production function, which we then use to construct an instrumental variable for D to be used in estimating the structural infant health (Eqn. 5). We therefore also estimate the model using predicted values of depression as a way of dealing with both the possible endogeneity of maternal depression and the limitations of our depression measure. One by-product of this research, then, is an empirical investigation into factors associated with maternal depressive symptoms, which have been overlooked in health economics research. (5)

   This theoretical framework clarifies the identifying restrictions used in the typical infant health model; income (I) and factors affecting the price of prenatal care (P), such as insurance status and community level variables, are used to identify prenatal care. (6) Warner (1998) discusses in detail his difficulty in finding satisfactory restrictions and notes that the structural birth weight equation may be weakly identified as a result. Such weak identification has been blamed in part for the lack of effectiveness of prenatal care found by many researchers (see, for example, Currie and Grogger 2000, who use policy changes in Medicaid and welfare as identifiers).

   We wish to isolate the effect of adding depression to the standard model of infant health, and so we employ typical identifying restrictions. In particular, we use a number of aggregate variables--health care price index aggregated to the state level, the ratio of Medicaid fees to private fee levels paid to OB-GYNs in the state (as in Gray 2001), population density, number of OB-GYNs per 1000 state population, n umber of general practitioners per 1000, and number of hospitals and HMOs per 1000. We also include some individual-level variables--whether the mother's prenatal care was paid for by Medicaid or private insurance and her income. As discussed in more detail in the results section, we subject the instrument sets to the usual tests, which they...