and Maternal Age among American Indian/Alaska Native Mothers: A Test of the Weathering Hypothesis

Jeff A. Dennis, Ph.D. Dept. of Internal Medicine, Texas Tech University Health Sciences Center, 3601 4th St., MS 9410, Lubbock, TX 79430 [email protected]

Abstract

Substantial research has examined birth outcomes by race/ethnicity, noting not only disparities by race/ethnicity, but different maternal age patterns in low birth weight (LBW) prevalence. The weathering hypothesis asserts that these different patterns result from cumulative exposure to socioeconomic disadvantage, found primarily among African American women, whose prevalence of LBW increases from the teenage years, through the 20s and 30s. Fewer studies have examined how mothers from other racial/ethnic groups compare to white and African American mothers, although indications are that the pattern expected by the weathering hypothesis do not consistently emerge in Latino mothers. An understudied group under this framework is American Indian and Alaska Native (AIAN) mothers, whose LBW prevalence is below the national average, but has not been examined by maternal age. Prior work has hypothesized that AIAN mothers should exhibit LBW age patterns similar to those seen in as a result of exposure to cumulative stress, trauma, and socioeconomic disadvantage, but this has not been empirically tested. This paper uses data from the 2000-2004 U.S. Birth File, which contains records of all U.S. births for those years to examine maternal age patterns in birth weight among AIAN mothers. Importantly, this study also considers high birth weight (HBW) births, given high prevalence of Type II diabetes in the AIAN population, and proposes that if “weathering” is occurring in this population, HBW prevalence likely will influence the observed maternal age patterns in birth weight, such that prevalence of births in normal range may resemble African Americans, even if LBW prevalence does not.

Introduction and Literature Review

Disparities in birth outcomes are an important marker of population health, given that they have implications for both maternal and child health. Racial and ethnic disparities in low birth weight (LBW) in the United States are well established. African

American mothers exhibited LBW prevalence about 1.8 times higher than white mothers in 2013 (Martin et al. 2015). Beyond these disparities, a body of research has explored a pattern termed the “weathering hypothesis,” which is represented by findings of disproportionately increasing rates of LBW with maternal age among African American relative to white mothers (Geronimus 1992, 1996; Reichman and Pagnini 1997; Rich-

Edwards et al. 2003). In general, explanations suggest that exposure to cumulative stress associated with higher rates of socioeconomic disadvantage over the life course produce an accelerated aging, or weathering, in this population. This age pattern has also been explored in Mexican American mothers, with evidence suggesting weathering in , but less so for LBW (Collins, Rankin, and Hedstrom 2012; Powers 2013;

Wildsmith 2002). This paper extends exploration of potential weathering patterns to another population with high levels of socioeconomic disadvantage, American Indian and

Alaska Native (AIAN) mothers.

About 7.5% of AIAN births in 2013 were LBW, only slightly higher than the 7% of non-Hispanic white births (Martin et al. 2015). Despite this reasonably small disparity, the prevalence of LBW by maternal age has not been explored. That is, if long-term exposure to disadvantage is in fact the primary driver of the age patterns of unfavorable birth outcomes that exemplify the weathering hypothesis, AIAN mothers might exhibit similar patterns to African Americans, even if overall levels of LBW are much lower. Palacios and Portillo (2009) provide a long list of unfavorable health outcomes with high prevalence in the AIAN population, similarly noting socioeconomic disparities in the population. Further, they suggest that historical trauma and cumulative stressors relating to a population with high rates of suicide, alcohol, and disease morbidity have the potential to produce accelerated deterioration of health in AIAN women of childbearing age relative to other racial/ethnic groups.

The understanding of maternal age patterns in LBW prevalence for AIAN women is complicated by two factors with counterbalancing effects on birth weight. First, high rates of smoking during have been found among AIAN mothers, a proximate factor that is known to contribute to LBW. However, rates of Type II and gestational diabetes are high in AIAN populations, which are known to be associated with high birth weight (HBW, above 4,000g) births.

National prevalence of HBW in AIAN mothers is 9.8%, the highest among all reported racial/ethnic groups compared to a national average of 8.2%, although only slightly higher than the 9.6% among non-Hispanic white mothers (Martin et al. 2015).

Risks of HBW to mother and infant are relatively lower than those for LBW births, although HBW is associated with unfavorable early life health outcomes as well as some later life outcomes such as diabetes and asthma (Harder et al. 2007; Orskou et al. 2003;

Remes et al. 2007; Stene et al. 2001)

Consideration of the effects of smoking and Type II Diabetes, taken as independent effects, would suggest that perhaps birth weight would be balanced between the countervailing effects such that LBW and HBW prevalence may not signal the AIAN population to be a particularly high risk group. As such, AIAN risk in birth outcomes may be masked somewhat in terms of birth weight, yet may manifest in other unfavorable outcomes. The standard outcomes of LBW or HBW may perhaps be too simplistic a measure to pick up birth outcomes risk in this population.

Hypothesis

H1: AIAN mothers will have disproportionately increasing prevalence of LBW births

with increasing age, relative to white mothers.

H2: AIAN mothers will have disproportionately lower prevalence of NBW births with

increasing age, relative to white mothers.

H3: AIAN mothers residing in more rural counties will have increased prevalence of

LBW with advancing age, relative to AIAN mothers in the same state in non-rural

counties

Data & Methods

Data for this study come from the 2000-2004 U.S. Birth File, which contains records of all births in the U.S. for those years. The data is collected and compiled by the

National Center for Health Statistics and downloaded from the National Bureau of

Economic Research (NBER.org 2015; NCHS 2000-2004). Each year contains over 4 million births (check), although given that the focal group of interest is relatively small, five years of data are combined for more robust comparisons, particularly by maternal age. 2004 is the last year that location data is available the birth files, so it was combined with the five preceding years.

The number of first births to AIAN mothers during this time frame is 53,957. The focal dependent variables are defined by standard cutoffs for LBW (less than

2500g, about 5.5 lbs) and HBW (over 4000g, about 8.8 lbs.) (Harder et al. 2007; Kramer

1987). A category of “normal birth weight” (NBW) is also created to designate births that fall between the low and high birth weight births (2500-4000g).

Analyses are limited to births recorded in the vital statistics file as the mother’s first birth, as parity is known to be positively associated with birth weight. A number of past studies on the weathering hypothesis have limited analyses to this subpopulation

(Geronimus 1996; Nabukera et al. 2009).

The public use birth files provide county FIPS codes for all U.S. counties with populations 100,000 or above, while counties with smaller populations are aggregated into a single code. This aggregation prevents identification of counties with Native

American reservations. This study approximates differences in reservation AIAN populations by examining rural counties in the states with large reservation populations:

Alaska, Arizona, New Mexico, South Dakota, Montana, North Dakota, Oklahoma, and

Utah, compared to larger populated counties in those states. This comparison is an imperfect approximation, but allows for consideration of potential different outcomes among AIAN populations residing on reservations compared to those in more populated areas. This subpopulation is comprised of 17,527 AIAN mothers in rural counties and

4,678 AIAN mothers in non-rural counties.

U.S.-born White and African American mothers are retained for comparison purposes, resulting in very large population subsamples of 3,637,830 and 765,832, respectively. Foreign-born mothers are excluded from these groups given existing evidence of an immigrant health advantage that may bias results. Maternal ages are grouped into 5 year categories, with a few exceptions, including “under 15,” 15-17”, “18-

19” and “35 plus”. Ages 15-19 are separated into two groups given that major social differences exist for having a child before finishing high school, typically at age 18.

Additional research suggests that a 15 year old mother may be less biologically equipped for childbearing than an 18 or 19 year old mother (Chen et al. 2007). The number of first births at age 40 plus is not large enough for statistical comparisons of this category.

Results

The use of large vital statistics birth files provides large sample sizes of race and maternal age for comparison of population birth weights. Table 1 shows LBW prevalence in the 5 year sample period for first births, among the three groups of interest.

AIAN birth weights in this table do not differ greatly in magnitude from white mothers at any age in this table. However, the weathering hypothesis poses that prevalence of unfavorable birth outcomes is accelerated with age. As such, the two columns on the right hand side of Table 1, followed by Figure 1, demonstrate that AIAN LBW prevalence worsens with age, relative to white mothers. The gradient of this difference is similar to that of African American mothers, despite the magnitude of the difference being much larger for African American mothers.

Table 1 & Figure 1 about here

Although the absolute differences are small, the differences in LBW between 15-

19 year old teenage mothers and their 30-34 year old counterparts provide interesting racial/ethnic differences. The white teenage mother group exhibits higher LBW prevalence than the 30-34 year old group, by 1%, whereas African American and AIAN

30-34 year old mothers have a 2% higher prevalence of LBW than their 15-19 year old counterparts. White mothers’ LBW prevalence, as has been well established in past work, begins very high in teenage years and drops to its lowest point at ages 25-29. For

African American and AIAN mothers, this drop happens earlier, at 20-24 or is practically non-existent, in the case of AIAN mothers, who see no improvement in LBW prevalence from ages 15-19 to 20-24. Subsequent analyses (not shown) indicate that 18-19 year old

AIAN mothers have a slightly lower LBW prevalence than those aged 20-24.

The descriptive findings above provide modest support for hypothesis 1, which posits that AIAN birth outcomes will become increasingly unfavorable with age, relative to those of whites. However, despite some evidence of patterns supportive of the weathering hypothesis, overall LBW prevalence is relatively low for AIAN mothers relative to African American mothers. This finding leads to the second hypothesis, which suggests that consideration of HBW may provide better insight into the risk profile of

AIAN birth outcomes.

Table 2 shows that, as expected based on national rates, HBW prevalence is higher for AIAN mothers than white and African American mothers at all ages, although most notably at the younger ages on the spectrum. Table 3 produces prevalence for NBW births by race and maternal age. Findings suggest that AIAN and African American mothers have disproportionately lower prevalence of NBW births relative to white mothers with increasing age. Figure 2 plots the differences between the three racial groups by age, showing that although AIAN mothers have slightly higher prevalence of births in the NBW range at younger ages, this advantage disappears by the age of 20.

Again, absolute differences are reasonably small, but provide modest support of hypothesis 2 in the context of NBW outcomes. As such, the weathering hypothesis may be better framed for AIAN mothers in the context of a more broad spectrum of birth outcomes beyond LBW.

Tables 2 & 3 & Figure 2 about here

Table 4 shows comparisons between AIAN mothers in 8 selected U.S. states with relatively large reservation populations, comparing mothers grouped by County FIPS code into a rural (counties with less than 100,000 residents) and non-rural (100,000+ residents) group. Mothers in the rural category, which encompasses a substantial portion of the U.S. AIAN population who reside on reservations (although not exclusively), are generally doing worse on birth outcomes relative to those mothers in the non-rural counties. However, as demonstrated by Figure 3, these unfavorable outcomes are reasonably constant with advancing age. That is, the sample does not produce evidence of an accelerated aging, or weathering in the rural population, and Hypothesis 3 is not supported. Disparities are reasonably constant with increasing age.

Table 4 & Figure 3 about here

Discussion

This paper sought to explore whether LBW prevalence patterns consistent with the weathering hypothesis are found among AIAN populations, as had been theorized by

Palacios and Portillo (2009). Findings indicate that although AIAN birth outcomes are reasonably similar to white mothers in aggregate, divisions by maternal age suggest increasingly unfavorable birth outcomes with advancing age. As such, there is evidence that weathering is present in this population, and as such, AIAN mothers of advancing age have increased risk in birth outcomes relative to white mothers. Subsequent analyses aimed to determine how the AIAN population residing on reservations may impact these patterns, that is, if those on reservations have experienced the most unfavorable socioeconomic conditions over their life course and therefore have more clear evidence of weathering. Although this rural population in states with substantial numbers of AIAN individuals on reservations exhibited somewhat less favorable birth outcomes than their non-rural counterparts, there was no support for the hypothesis that weathering would be most pronounced in this population.

Limitations

Public use birth files do not allow for the identification of mothers residing on

AIAN reservations. Substantial evidence suggests that disadvantage in the AIAN population may be most pronounced on reservations, and as such, birth outcomes would be expected to be more unfavorable. This study approximated this comparison, but access to restricted data would likely provide a clearer picture of this contrast.

Future analyses should examine second and later births, as weathering as a mechanism of accelerated aging should not be limited to women who are having their first child at later ages. This line of inquiry also relates to integrating an understanding of how normative age at first birth factors into the birth outcome risk of a population. Both

Native American and African American mothers have lower average age at first birth than whites, and similarly have less favorable birth outcomes at earlier ages. This relationship should be explored more fully.

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Table 1. Prevalence of LBW, by Race, among U.S.-born first births, 2000-2004 White/Black White/AIAN Maternal age White Black AIAN difference difference under 14 0.11 0.16 0.08 0.04 -0.04 15-19 0.08 0.13 0.07 0.05 -0.01 20-24 0.07 0.12 0.07 0.06 0.00 25-29 0.06 0.13 0.08 0.07 0.02 30-34 0.07 0.15 0.09 0.08 0.01 35 plus 0.10 0.19 0.12 0.09 0.02 Source: NCHS 2000-2004 *LBW - <2500g

Table 2. Prevalence of HBW, by Race, among U.S.-born first births, 2000-2004 White/Black White/AIAN Maternal age White Black AIAN difference difference under 14 0.04 0.02 0.07 -0.03 0.03 15-19 0.07 0.03 0.09 -0.04 0.02 20-24 0.09 0.04 0.10 -0.05 0.01 25-29 0.10 0.05 0.11 -0.05 0.01 30-34 0.10 0.06 0.11 -0.05 0.00 35 plus 0.10 0.05 0.11 -0.05 0.01 Source: NCHS 2000-2004 *HBW - >4000g

Table 3. Prevalence of NBW, by Race, among U.S.-born first births, 2000-2004 White/Black White/AIAN Black/AIAN Maternal age White Black AIAN difference difference difference under 14 0.84 0.83 0.85 -0.01 0.01 0.02 15-19 0.85 0.84 0.85 -0.01 -0.01 0.01 20-24 0.85 0.84 0.84 -0.01 -0.01 0.00 25-29 0.84 0.81 0.81 -0.02 -0.02 0.00 30-34 0.82 0.79 0.81 -0.03 -0.02 0.02 35 plus 0.80 0.76 0.77 -0.04 -0.03 0.01 Source: NCHS 2000-2004 *NBW - 2500g to 4000g

Table 4. AIAN birth weight prevalence, rural vs. non-rural counties in states* with higher reservation populations LBW HBW NBW Rural Non-rural Rural Non-rural Rural Non-rural Maternal age AIAN AIAN AIAN AIAN AIAN AIAN under 14 0.05 0.08 0.07 0.04 0.88 0.88 15-19 0.06 0.05 0.09 0.08 0.85 0.87 20-24 0.07 0.06 0.11 0.08 0.83 0.86 25-29 0.08 0.06 0.12 0.11 0.80 0.83 30-34 0.10 0.10 0.12 0.10 0.78 0.79 35 plus 0.11 0.10 0.09 0.12 0.80 0.78 Source: NCHS 2000-2004 *States include Alaska, Arizona, Montana, New Mexico, North Dakota, Oklahoma, South Dakota, Utah