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Poverty and Infant Mortality -- United States, 1988

Although previous reports have documented that rates of low birthweight and intrauterine growth retardation are higher among infants of women living in poverty (1), the infant mortality risk among infants born to women with low incomes has not been characterized recently. To analyze the relation between parental low income and infant mortality, CDC analyzed data from the 1988 National Maternal and Infant Health Survey (NMIHS) (the most recent data available). This report presents the findings of the analysis and indicates that for women with household incomes below the poverty level * in 1988, the infant mortality rate was 60% higher and the postneonatal mortality rate was twice as high as those for women living above poverty level.

NMIHS was a national population-based survey that collected data about pregnancy outcomes from vital records and questionnaires administered by mail and telephone. The survey had two stratified systematic samples: 13,417 women who had had a live-born infant in 1988 and 8166 women whose infant had died within 1 year after birth (2). Of the mothers in the live-born infant sample and the infant death sample, 9953 (74%) and 5332 (65%), respectively, participated in the survey. Data on household income and household size from the NMIHS maternal questionnaire were used to classify women as living below or above the U.S. poverty threshold (in 1988: $12,092 for a family of four) (3). Because previous studies consistently demonstrated relatively high risks for adverse pregnancy outcomes among blacks, black infants were oversampled in NMIHS to allow for more detailed analysis of this group (4). Data are presented only for blacks and whites because numbers for other racial/ethnic groups were too small for meaningful analysis.

An infant death was defined as the death of a live-born infant before his or her first birthday; a neonatal death, as the death of a live-born infant less than 28 days after birth; and a postneonatal death, as the death of a live-born infant 28-364 days after birth (4). Data were statistically weighted to reflect the number of live births and infant deaths in the United States in 1988, and mortality rates were computed as estimates of the number of deaths per 1000 live births. Risk ratios were calculated by dividing the mortality rate for infants born to women living below the poverty threshold by the mortality rate for infants born to women living above the poverty threshold. Confidence intervals (CIs) for the risk ratios were computed using SUDAAN (5). Mortality rates and risk ratios for the relation of income to mortality were calculated for six sociodemographic and behavioral variables that consistently have been associated with both poverty and infant mortality (4): marital status, maternal age, cigarette smoking during pregnancy, timing of the first prenatal-care visit, maternal educational attainment, and race of the mother.

Based on NMIHS, overall, 20.1% of the women who delivered live-born infants in 1988 reported a household income below the poverty level (Table_1). However, the percentage of women living below poverty level varied substantially in relation to specific risk characteristics (e.g., 6.8% of mothers with greater than 12 years of education compared with 45.8% of those with less than or equal to 11 years). Overall, 13.9% of mothers did not report information about household income.

In the total 1988 NMIHS birth population, the overall estimated infant mortality rate was 10.0 per 1000 live-born infants. However, the rate varied by poverty level and was 8.3 for infants of women with incomes above poverty level and 13.5 for women with incomes below poverty level (risk ratio {RR}=1.6 {95% CI=1.5-1.8}) (Table_2). The overall association between poverty and infant mortality was stronger for postneonatal deaths (RR=2.0 {95% CI=1.8-2.3}) than for neonatal deaths (RR=1.4 {95% CI=1.3-1.6}).

Compared with the rates for infants of mothers living above poverty level, the rates for infants of mothers living below poverty level were consistently higher among women who were married, aged greater than or equal to 18 years, received prenatal care during the first trimester of pregnancy, had greater than or equal to 12 years of education, and were either smokers or nonsmokers (Table_2). There was no relation between poverty and mortality rates for infants born to mothers aged less than or equal to 17 years or mothers who did not receive prenatal care during the first trimester of pregnancy. The association between poverty and infant mortality was stronger for infants born to white women (RR=1.5 {95% CI=1.3-1.7}) than for those born to black women (RR=1.1 {95% CI=1.0-1.3}). For blacks, while neonatal death rates were similar for infants born to women living above or below poverty level (RR=0.9 {95% CI=0.8-1.1}), postneonatal death rates were higher for infants of women living below poverty level (RR=1.6 {95% CI=1.4-1.9}).

Reported by: Infant and Child Health Studies Br, Div of Health and Utilization Analysis, National Center for Health Statistics, CDC.

Editorial Note

Editorial Note: An association between poverty and risk for increased infant mortality in the United States was first noted in the early 1900s (5). However, information about income has not been routinely available on U.S. birth or death certificates, and estimates of the excess risk for death among infants born to women living in poverty can be derived primarily from special surveys such as the NMIHS (1,5).

The findings in this analysis of NMIHS indicate that, for infants born to women living in poverty in the United States in 1988, overall excess mortality risk was approximately 60% compared with infants born to women living above the poverty level. Although higher proportions of women living below poverty level than above were in high-risk groups (i.e., unmarried, adolescent, smokers, black, or had had late or no prenatal care), the higher risk for death among infants born to women living below the poverty level was not associated with these maternal characteristics. Instead, the effect of poverty was stronger for infants born to women who were otherwise at low absolute risk for infant mortality (i.e., women who were married, aged greater than or equal to 18 years, nonsmokers, white, had had early prenatal care, or with greater than or equal to 12 years of education). One possible explanation for this is that mortality rates for infants born to high-risk women are already so high -- even among those living above poverty level -- that poverty has little additional effect.

The NMIHS data also indicate that the relation of poverty to mortality was especially strong for postneonatal death, with approximately a twofold excess risk for postneonatal death among infants of women living in poverty. This finding is consistent with the established concept that environmental factors are more often associated with postneonatal deaths than with neonatal deaths (4,6-8).

The findings in this report are subject to at least two limitations. First, the non-response rates for mothers among the live-born infant sample and the infant death sample were substantial (26% and 35%, respectively); however, postsampling adjustments were made to account for nonresponse (2). Second, because 14% of NMIHS respondents did not report information about household income -- and the percentage was even higher for relatively high-risk groups (Table_1) -- the relation between poverty and infant mortality may have been biased. Although the direction of this potential bias is unknown, its magnitude is probably relatively small.

A high proportion of the increased risk for death among infants born to women living in poverty reflects an excess of postneonatal deaths, many of which are caused by infectious etiologies or injuries (7) and which can be prevented by medical care and public health interventions. Strategies to reduce the excess risk for postneonatal death in low-income families include increasing the availability of health care in medically underserved areas and removing the financial barriers to health care. In addition, improved access to health care should be linked to education and community-oriented programs to inform parents about preventive measures for infections (e.g., rehydration for diarrhea) and injuries (e.g., child restraints and smoke detectors). Identification of infants at high risk for postneonatal death can assist in ensuring that such infants receive adequate health care; this strategy was used to reduce postneonatal death rates in a statewide intervention program in West Virginia, in which high-risk infants were linked with primary-care physicians who provided specified care plans (9).

The Public Health Service recently highlighted the need for improved characterization of inequalities in health (10), and in September 1994, participants at a conference sponsored by the National Institutes of Health concluded that research efforts should focus on the mechanisms that link social and economic disparities to health (10). The findings in this report suggest that planning efforts for maternal and child health programs should include consideration of low income, in addition to other social and behavioral characteristics, such as adolescent childbearing, cigarette smoking during pregnancy, access to prenatal care, low maternal educational attainment, and race/ethnicity.

References

  1. Parker JD, Schoendorf KC, Kiely JL. Associations between measures of socioeconomic status and low birth weight, small for gestational age, and premature delivery in the United States. Annals of Epidemiology 1994;4:271-8.

  2. NCHS. National Maternal and Infant Health Survey {Machine-readable public-use data tape}. Hyattsville, Maryland: US Department of Health and Human Services, Public Health Service, CDC, 1991.

  3. Bureau of the Census. Poverty in the United States, 1988 and 1989. Washington, DC: US Department of Commerce, Economics and Statistics Administration, 1991. (Current population reports; series P60, no. 171).

  4. Wilcox LS, Marks JS, eds. From data to action: CDC's public health surveillance for women, infants, and children. Atlanta, Georgia: US Department of Health and Human Services, Public Health Service, CDC, 1994.

  5. Shah BV. Software for survey data analysis (SUDAAN) version 6.34 {Software documentation}. Research Triangle Park, North Carolina: Research Triangle Institute, 1993.

  6. Woodbury RH. Economic factors in infant mortality. J Am Stat Assoc 1924;19:137-55.

  7. Adams MM, Rhodes PH, McCarthy BJ. Postneonatal deaths from infections and injuries. Am J Prev Med 1991;7:166-71.

  8. Kessel SS. Postneonatal mortality: a performance indicator of the child health care system. Pediatrics 1990;86:1107-11.

  9. Myerberg DZ, Carpenter RG, Myerberg CF, Britton CM, Bailey CW, Fink BE. Reducing postneonatal mortality in West Virginia: a statewide intervention program targeting risk identified at or after birth. Am J Public Health 1995;85: 631-7.

  10. Lee PR, Moss N, Krieger N. Measuring social inequalities in health. Public Health Rep 1995;110:302-5.

Poverty statistics are based on a definition originated by the Social Security Administration in 1964, subsequently modified by federal interagency committees in 1969 and 1980, and prescribed by the Office of Management and Budget as the standard to be used by federal agencies for statistical purposes.
Table_1
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TABLE 1. Percentage distribution of infants born to women above and below the
poverty level *, by selected maternal characteristics -- National Maternal and Infant
Health Survey, United States, 1988 +
==========================================================================================
                                                    Household income
                                      --------------------------------------------
                              No.         Below          Above
                           live-born  poverty level  poverty level    Not reported
                            infants   --------------------------------------------
Maternal characteristic     (n=9953)   %    (SE &)     %     (SE)      %     (SE)
----------------------------------------------------------------------------------
Marital status
  Married                     5869    12.9   (0.5)    76.5   (0.7)    10.6   (0.5)
  Unmarried                   4084    41.0   (1.3)    35.5   (1.3)    23.5   (1.1)

Age (yrs)
  <=17                         679    42.3   (3.2)    29.7   (3.1)    28.0   (2.8)
  >=18                        9274    19.0   (0.5)    67.8   (0.6)    13.2   (0.5)

Cigarette smoking
  during pregnancy
  Yes                         2997    26.3   (1.1)    59.1   (1.3)    14.6   (0.9)
  No                          6956    17.4   (0.6)    69.0   (0.7)    13.6   (0.5)

First prenatal-care visit
  1st-3rd month
    of pregnancy              6903    14.9   (0.5)    72.6   (0.7)    12.5   (0.5)
  After 3rd month or none     3050    37.3   (1.3)    44.1   (1.4)    18.6   (1.1)

Education
  <=11 yrs                    2454    45.8   (1.5)    32.5   (1.5)    21.7   (1.2)
    12 yrs                    4098    20.5   (0.8)    65.4   (1.0)    14.1   (0.7)
   >12 yrs                    3401     6.8   (0.5)    83.4   (0.8)     9.8   (0.7)

Race @
  White                       4703    16.0   (0.6)    71.6   (0.8)    12.4   (0.6)
  Black                       4960    40.6   (0.8)    37.5   (0.8)    21.9   (0.7)

Total                         9953    20.1   (0.5)    66.0   (0.6)    13.9   (0.5)
----------------------------------------------------------------------------------
* In 1988, the poverty threshold was $12,092 for a family of four (3).
+ Most recent year for which data were available.
& Standard error.
@ Numbers for other racial/ethnic groups (n=290) were too small for meaningful analysis.
==========================================================================================

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Table_2
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TABLE 2. Estimated mortality rate * for infants born to women living above and below poverty level +, by age of infant at
death and selected maternal characteristics -- National Maternal and Infant Health Survey (NMIHS), United States, 1988 &
*********************************************************************************************************************************************************************************
                                         Household income                                                                            Household income
                                        ------------------                                                                          ------------------
                                         Below     Above                                                                             Below     Above
                             No.        poverty   poverty                                                                No.        poverty   poverty
Age of infant at death/    deaths @      level     level     Risk                           Age of infant at death/    deaths @      level     level     Risk
  Maternal characteristic  (n=4246)     (n=1677)  (n=2569)   ratio     (95% CI **)            Maternal characteristic  (n=4246)     (n=1677)  (n=2569)   ratio     (95% CI **)
---------------------------------------------------------------------------------------+---------------------------------------------------------------------------------------
<28 DAYS ++                                                                                   First prenatal-care visit
  Mother's marital                                                                              1st-3rd month
    status                                                                                        of pregnancy            834          4.4       2.3      1.9       (1.6-2.2)
    Married                  1680          7.1       5.1      1.4       (1.2-1.6)               After 3rd month
    Unmarried                1066          8.5       7.5      1.1       (0.9-1.3)                 or none                 663          7.4       5.5      1.3       (1.1-1.6)
  Maternal age (yrs) &&                                                                       Mother's education
    <=17                      188          9.1       9.0      1.0       (0.6-1.6)               <=11 yrs                  487          6.5       4.6      1.4       (1.1-1.8)
    >=18                     2553          7.6       5.4      1.4       (1.3-1.6)                 12 yrs                  574          5.0       2.8      1.8       (1.5-2.2)
  Cigarette smoking                                                                              >12 yrs                  436          4.8       2.5      1.9       (1.4-2.6)
    during pregnancy                                                                          Mother's race @@
    Yes                       889          8.5       5.8      1.5       (1.2-1.8)               White                     809          4.9       2.7      1.8       (1.5-2.2)
    No                       1857          7.3       5.3      1.4       (1.2-1.6)               Black                     641          7.1       4.4      1.6       (1.4-1.9)
  First prenatal-care visit                                                                   Total                      1497          5.7       2.8      2.0       (1.8-2.3)
    1st-3rd month                                                                      
      of pregnancy           1678          6.4       4.6      1.4       (1.2-1.6)           <1 YEAR
    After 3rd month                                                                           Mother's marital
      or none                1068          9.6      10.1      0.9       (0.8-1.1)               status
  Mother's education                                                                            Married                  2571         12.2       7.9      1.6       (1.4-1.8)
    <=11 yrs                  713          8.3       7.4      1.1       (0.9-1.4)               Unmarried                1675         14.6      11.0      1.3       (1.1-1.6)
      12 yrs                 1077          7.1       5.5      1.3       (1.1-1.5)             Maternal age (yrs) &&
     >12 yrs                  956          8.3       5.1      1.6       (1.3-2.1)               <=17                      314         15.7      13.7      1.1       (0.8-1.7)
  Mother's race @@                                                                              >=18                     3925         13.2       8.2      1.6       (1.5-1.8)
    White                    1413          6.4       4.9      1.3       (1.1-1.5)             Cigarette smoking
    Black                    1278         10.9      11.6      0.9       (0.8-1.1)               during pregnancy
  Total                      2746          7.8       5.5      1.4       (1.3-1.6)               Yes                      1502         16.0       9.4      1.7       (1.5-2.0)
                                                                                                No                       2744         11.8       7.9      1.5       (1.3-1.7)
28-364 DAYS ***                                                                               First prenatal-care visit
  Mother's marital                                                                              1st-3rd month
    status                                                                                        of pregnancy           2512         10.8       6.9      1.6       (1.4-1.8)
    Married                   888          5.1       2.7      1.9       (1.6-2.3)               After 3rd month
    Unmarried                 609          6.2       3.5      1.8       (1.4-2.2)                 or none                1734         17.0      15.6      1.1       (0.9-1.3)
  Maternal age (yrs) &&                                                                       Mother's education
    <=17                      126          6.6       4.6      1.4       (0.8-2.4)               <=11 yrs                 1200         14.8      12.0      1.2       (1.0-1.5)
    >=18                     1369          5.5       2.8      2.0       (1.7-2.3)                 12 yrs                 1654         12.1       8.3      1.5       (1.3-1.7)
  Cigarette smoking                                                                              >12 yrs                 1392         13.0       7.5      1.7       (1.4-2.2)
    during pregnancy                                                                          Mother's race @@
    Yes                       613          7.5       3.6      2.1       (1.7-2.5)               White                    2224         11.3       7.6      1.5       (1.3-1.7)
    No                        884          4.5       2.6      1.8       (1.5-2.1)               Black                    1920         17.9      16.0      1.1       (1.0-1.3)
                                                                                              Total +++                  4246         13.5       8.3      1.6       (1.5-1.8)
---------------------------------------------------------------------------------------+---------------------------------------------------------------------------------------
  * Per 1000 live-born infants.
  + In 1988, the poverty threshold was $12,092 for a family of four (3).
  & The most recent year for which data were available.
  @ Infant deaths for which maternal household income was not reported (n=1086) were excluded from this analysis.
 ** Confidence interval.
 ++ Neonatal death.
 && For seven infants, maternal age at the infant's death was unknown.
 @@ Numbers for other racial/ethnic groups were too small for meaningful analysis; for infants aged <28 days at death, 55 deaths were
    excluded from analysis; for those aged 28-364 days at death, 47; and for those aged <1 year at death, 102.
*** Postneonatal death.
+++ For three infants, age at death was unknown.
*********************************************************************************************************************************************************************************




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