pISSN 0513-5796   eISSN 1976-2437
Open Access, Peer-reviewed, Monthly
    Yonsei Med J. 2020 Sep;61(9):805-815. English.
    Published online Aug 27, 2020.
    https://doi.org/10.3349/ymj.2020.61.9.805
    © Copyright: Yonsei University College of Medicine 2020
    Original Article

    Role of Parental Social Class in Preterm Births and Low Birth Weight in Association with Child Mortality: A National Retrospective Cohort Study in Korea

    Mia Son,1 Soo-Jeong An,2 Seung-Ah Choe,3 Mijin Park,4 and Young-Ju Kim5
      • 1Department of Preventive Medicine, School of Medicine Kangwon National University, Chuncheon, Korea.
      • 2Department of Benefits Strategy, National Health Insurance Service, Wonju, Korea.
      • 3Department of Preventive Medicine, Korea University Medical College, Seoul, Korea.
      • 4Department of Environmental Health Sciences, Graduate School of Public Health, Seoul National University, Seoul, Korea.
      • 5Department of Statistics, College of Business Administration, Kangwon National University, Chuncheon, Korea.
    • Co-corresponding author: Mijin Park, PhD, Department of Environmental Health Sciences, Graduate School of Public Health, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea. Tel: 82-2-880-2813, Fax: 82-2-745-9104, Email: r24850@snu.ac.kr
      Co-corresponding author: Young-Ju Kim, PhD, Department of Statistics, College of Business Administration, Kangwon National University, 1 Kangwondaehak-gil, Chuncheon 24341, Korea. Tel: 82-33-250-8431, Fax: 82-33-259-5663, Email: ykim7stat@kangwon.ac.kr
    Received February 11, 2020; Revised July 15, 2020; Accepted August 06, 2020.

    This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    Abstract

    Purpose

    We explored the role of parental social class in preterm birth (PTB) and low birth weight (LBW) in association with child mortality in Korea.

    Materials and Methods

    A total of 7,302,732 births in Korea between 1995 and 2007 were used for designing the national retrospective cohort study. Kaplan-Meier survival curves and Cox proportional hazard models were used to determine the risk of child death after adjusting for covariates.

    Results

    Parental social class was associated with adverse birth outcomes and child mortality in Korea. Parental social class increased the strength of the relationship of adverse birth outcomes with child mortality. Child mortality was higher among PTB and LBW infants from parents with a lower social class than normal births from parents with a higher social class. In particular, the disparity in child mortality according to parental social class was greater for LBW and PTB than intrauterine growth retardation births. When one of the parents had a middle-school education or lower, the disparity in child mortality due to adverse birth outcomes was large regardless of the other spouse's educational status. Inactive economic status for the father, as well as an occupation in manual labor by the mother, increased the risk of child mortality.

    Conclusion

    Strong relationships for social inequalities and adverse birth outcomes with inequalities in child mortality in South Korea were found in this study. Tackling social inequalities, as well as reducing adverse birth outcomes, are needed to reduce the disparities in child mortality in South Korea.

    Keywords
    Social class differences; child mortality; parental social class; parental education; parental employment; low birth weight; preterm birth

    INTRODUCTION

    Adverse birth outcomes, such as low birth weight (LBW),1, 2 preterm birth (PTB),1, 2 and intrauterine growth retardation,3, 4 are known risk factors for infant or child mortality, as are socioeconomic factors.5, 6, 7, 8, 9 Whereas most studies have treated adverse birth outcomes and socioeconomic factors as risk factors for infant or child mortality separately,6, 7, 10, 11, 12, 13, 14, 15, 16 few investigations of the interactive effects of social class and adverse birth outcomes on child mortality have been reported. A previous study reported that parental education, the type of parental work, and employment were interactively related to adverse birth outcomes.14 The most vulnerable populations in terms of adverse birth outcomes were those performing manual work or in an economically inactive state with a middle school education or lower.14 However, the relationship of these factors on child mortality has not been addressed. Therefore, to what extents the interactive effects of social class and adverse birth outcomes impact the mortality and survival of infants and children need to be investigated.

    This study aimed to investigate the role of parental social class in the relationship of adverse birth outcomes with child mortality. We hypothesized that a combination of social factors and adverse birth outcomes would increase child mortality. The primary aims of this study were to determine how parental social class and adverse birth outcomes interact and to determine the effects of this interaction on child mortality in Korea in a cohort of births from 1995–2007.

    MATERIALS AND METHODS

    Data for retrospective cohort study

    A Korean national retrospective birth cohort of 7,344,797 infants was constructed by linking national birth and death registration records to identify deaths among all births according to individual 13-digit social security numbers collected by the Korean National Statistics Office between 1995 and 2007 and followed up to the calendar year 2007. Records deemed to be invalid were excluded, and the resulting overall matched birth cohort encompassing the period 1995–2007 was comprised of 26162 deaths among 7,302,732 births (99.43% of the total number of births during that period). The total number of person-years during this period was 52,117,426.33.

    The survival status of the study cohort born in the period 1995–2007 was identified from the date of birth (as early as January 1, 1995) to December 31, 2007. It was assumed that all cohort members not found among the death certificate files were alive at the end of the study period. The survival time was calculated as the time from date of birth to date of death. The survival time of children not identified as dead was calculated as the time from the date of birth to the end of the study period.

    Variables

    Information on the variables used in this study was collected from the birth registry records based on birth certificates and the death registry records based on death certificates.

    Parental education and employment status were used as indicators of social class. Parental education was stratified into less than elementary school (≤6 years), middle school (7–9 years), high school (10–12 years), and university or higher (≥13 years). Parental employment status was stratified into non-manual (e.g., legislators, senior officials, managers, professionals, technicians and practical professionals, office workers, service workers, and sales workers), manual (e.g., skilled agricultural, forestry, and fishery workers, craft workers, device and machine operators and assemblers, and laborers), and economically inactive (e.g., unemployed, students, housekeepers, and soldiers).

    With respect to birth characteristics, data regarding infant sex, parental age at childbirth, gestational age at childbirth, birth weight, multiple births, parity, and history of the death of a previous child were included. Parental age at childbirth was categorized into the following 5-year groups: ≤24, 25–29, 30–34, 35–39, or ≥40 years. Adverse birth outcomes were defined as LBW (birth weight <2500 g), PTB (gestational age <37 weeks), PTB-LBW (both gestational age <37 weeks and birth weight <2500 g). Multiple births and parity were classified as 1, 2 and ≥3, and the history of the death of a previous child was dichotomized as yes or no. Variables pertaining to the date and cause of death were obtained from death certificates.

    Statistical methods

    The crude death rates, age-adjusted death rates, the incidence of mortality, and the number of person-years were calculated. The probability of surviving up to 13 years of age was estimated using the Kaplan-Meier method. Kaplan-Meier survival curves were used to describe the pattern of survival within early childhood for the levels of each study variable. Cox proportional hazards regression was used to examine the associations among gestational age, birth weight, and parental social class and infant and child mortality, adjusting for covariates (infant sex, maternal age, parental age, multiple births, maternal parity, death of previous children, and year of birth). Child mortality in this study was defined as death from the age of 0 to 13 years of age.

    The interplay between parental social class and adverse birth outcomes and its effects on mortality were examined by calculating hazard ratios (HRs) for interactions between parental social class and adverse birth outcomes after adjusting for covariates. Two- and three-way interactions between parental social class and adverse birth outcomes with child mortality were investigated through an interaction test. The likelihood ratio statistic was used to test for interactions between the risk factors.

    SAS (version 9.3, SAS Institute, Cary, NC, USA) was used for all analyses. The threshold for statistical significance was set at p<0.05 for the main effects.

    This study was approved by the Institutional Review Board of Kangwon National University Hospital (KNUH-2019-11-002).

    RESULTS

    The role of combined social class on child mortality

    An effect of social inequality on child mortality was found in this study. Births to parents with education levels of high school and middle school or lower were associated with higher child mortality than in parents with a university education or higher. The effect of parental educational level on child mortality was stronger than that of parental employment status (Table 1).

    Table 1
    The Difference in Child Mortality According to Parental Education and Employment Status, PTB and LBW

    Child mortality was also affected by adverse birth outcomes. Births with adverse birth outcomes (PTB, LBW) had a higher risk of death than normal births (Table 1).

    The combined parental social class showed an adverse linear relationship with child mortality from higher to lower social class. Child mortality was highest among fathers with middle school education or lower, as well as those in the economically inactive group. Child mortality was highest among mothers with middle school education level or lower, as well as those engaged in manual work, compared to mothers who were economically inactive (Table 2).

    Table 2
    Interactive Effects of Parental Education and Parental Employment on Child Mortality

    The role of effect modification by social class on the relationship of adverse birth outcomes with child mortality

    Significant interactions were found between parental education or employment and adverse birth outcomes with child mortality (log-likelihood test, p<0.001) (Tables 2, 3, 4, 5). A lower social class among parents increased the association between adverse birth outcomes and child mortality. The adjusted HRs of births with adverse birth outcomes relative to those of normal births increased when parental education level was lower and when parental employment status was economically inactive or manual labor. The disparity in child mortality between normal births and LBW was the greatest for fathers with a middle-school education or lower {HR=9.62 [95% confidence interval (CI): 8.78, 10.53]}, followed by fathers with a high school education [HR= 7.91 (95% CI: 7.54, 8.31)] and fathers with a university education [HR=7.03 (95% CI: 6.67, 7.42)]. The results for mothers were similar [HR=9.64 (95% CI: 8.76, 10.53); HR=7.73 (95% CI: 7.37, 8.11), and HR=7.51 (95% CI: 7.07, 7.97), respectively] (Table 3). The adjusted HRs were larger for LBW than for PTB births and thus, the interactive effects between parental education and adverse birth outcome were greater for LBW than for PTB births (Table 3, Supplementary Table 1, only online). The interactive relationships between parental employment status and adverse birth outcomes with child mortality were similar to but weaker than those between parental education and adverse birth outcomes with child mortality (Table 4). The disparity in child mortality between adverse births and normal births was greater when the fathers were economically inactive and the mothers were engaged in manual employment.

    Table 3
    Interactive Effects of Parental Education, PTB and LBW on Child Mortality

    Table 4
    Interactive Effects of Parental Employment Status, PTB and LBW on Child Mortality

    Table 5
    Three-Way Interactions of Paternal Education and Maternal Education with LBW, and Paternal Employment and Maternal Employment with LBW on Child Mortality

    Kaplan-Meier survival curves showed that the gap in survival curves was higher for adverse births from parents with a lower level of education or parents with manual employment or in an economically inactive status, compared to normal births from parents with higher education or with non-manual employment status. The disparity in survival rates according to parental social class was greater for PTB-LBW or LBW than for PTB births.

    The role of combined social class on the relationship of adverse birth outcomes and child mortality

    Three-way interactive analysis showed that combined lower social class for parents increased the relationship between adverse birth outcomes and child mortality (Table 5). The differences in child mortality between normal births and LBW were greatest for fathers with a middle-school education or lower and mothers with a university education or higher [HR=14.18 (95% CI: 9.41, 21.37)], followed by a middle-school education or lower for both parents [HR=10.95 (95% CI: 9.72, 12.33)], fathers with a university education or higher, and mothers with a middle-school education or lower [HR=10.56 (95% CI: 5.99, 18.61)] (Table 5).

    Regarding employment status, the results were similar to those for education level, although the strength of the relationship was greater. The disparity was greatest for economically inactive fathers and mothers in manual employment [HR=15.23 (95% CI: 8.83, 26.29)], followed by economically inactive fathers and mothers in non-manual employment [HR=12.29 (95% CI: 8.27, 18.27)] and economically inactive parents [HR=10.45 (95% CI: 9.19, 11.88)] (Table 5).

    Interestingly, the disparity in child mortality between adverse birth outcomes was greater when one of the parents had lower social status (e.g., middle school education or lower and economically inactive or manual work). When one of the parents had a middle-school education or lower, the disparity in child mortality due to LBW was larger regardless of the spouse's level of education. The father's education level had more effect than the mother's on child mortality. The results for employment status were similar. The disparity was larger when the father was economically inactive, regardless of the maternal employment status. A father's inactive economic status had a greater effect than the mother's on child mortality in general. However, child mortality was highest for a combination of an economically inactive father and an occupation in manual labor for the mother (Table 5).

    Kaplan-Meier survival probabilities showed a gap in survival curves for a combination of PTB and LBW. The disparity was greater for parents with a middle-school education or lower and fathers in an economically inactive state and mothers employed in manual labor (Figs. 1 and 2).

    Fig. 1
    Kaplan-Meier survival probabilities according to the combination of gestational age and LBW, stratified according to parental education level. PTB, preterm birth; LBW, low birth weight.

    Fig. 2
    Kaplan-Meier survival probabilities according to the combination of gestational age and LBW, stratified according to parental employment status. PTB, preterm birth; LBW, low birth weight.

    DISCUSSION

    The key findings of this study were the effects of social inequalities on child mortality and the individual and interactive effects of parental social class (parental educational level and employment status) and adverse birth outcomes with child mortality. The interaction between parental social class and adverse birth outcomes with child mortality was stronger than that for other factors. The differences in child mortality between normal births and adverse births were greatest for parents with lower levels of education (middle school education or lower) and when the fathers were economically inactive and the mothers were engaged in manual employment. Child mortality in adverse births was predominantly influenced by one parent's lower social class, regardless of the spouse's social class.

    This study showed that differences in parental social status can affect child mortality. Child mortality was higher among births from lower educated parents, economically inactive fathers, and mothers in employed in manual labor, concurring with the results of previous studies in Korea12, 13, 15, 16 and worldwide.17, 18, 19, 20, 21 This study also showed a linear relationship between combined parental social class (parental education and employment) and child mortality. This result was similar to a previous study,14 although the effect on child mortality in this study was greater than that of adverse birth outcomes in the previous study. The association between lower parental social class and higher child mortality suggests that parental social class has a stronger effect on child mortality than adverse birth outcomes.

    We found that parental social classes (parental education and employment) interactively affected child mortality. Child mortality was higher when both parents had lower educational levels and when the father was economically inactive and the mother performed manual labor. These results are consistent with those of a previous study,14 wherein adverse birth outcomes were higher when both parents had lower education levels and when the father was economically inactive and the mother was employed in manual labor.14 However, the strength of the relationship between social class and child mortality was greater in this study than adverse birth outcomes in the previous study.

    Our results imply that the lower the parents' social class, the higher the child mortality. A lower social class might influence material conditions, as well as social relationships, resulting in adverse birth outcomes and child mortality. Therefore, social status differences may be an area to address with respect to child mortality, specifically the lowest social class (lower education as well as lower employment status, such as economically inactive and manual employment).22, 23, 24, 25, 26

    In this study, significant interactions were noted between parental social class factors associated with adverse births (PTB and LBW) and child mortality. The findings of the present study imply that parents from a lower social class could be a factor in fetal undernutrition, resulting in disproportionate fetal growth, leading to LBW and PTB and, thus, increasing child mortality associated with social status differences in child mortality. The risk factors for LBW or PTB were socioeconomic situations, maternal nutrition, maternal disease, pre-pregnancy testing, lifestyle, and drinking/smoking, which are all closely related to parental social class.27, 28, 29, 30, 31, 32 Thus, socioeconomic cultural conditions, to some degree, may underlie the risks of adverse birth outcomes.29, 33 Therefore, these conditions could also affect the association between the parents' social class and adverse birth outcomes resulting in child mortality.

    An interesting finding of our study was that child mortality for adverse births was predominantly influenced by one parent with lower social status, regardless of the spouse's social class. The disparity in child mortality between normal births and adverse births was greater when one parent had a lower social class, middle school level or lower education, and economically inactive or manual work. This suggests that one of the parents might influence the entire family's social class. In particular, the father's social class can be a social class determinant for the family, the dominant risk factor for child mortality.

    Another important finding in this study is that when fathers are in an economically inactive state, the disparity in child mortality between normal births and adverse births became higher when the mothers were engaged in manual work. When the father was economically inactive, the mother might be employed in order to maintain the household income (the added-worker effect) or she might recognize that the labor market and the economy are in a difficult state and stop looking for work, thus falling into an economically inactive state (the discouraged-work effect).22, 23, 24, 25 Ultimately, the spouse's economically inactive status forces the mother to either search for a job or to be unemployed, which are detrimental to maternal health and thus, also to a newborn's health. Economically inactive fathers also push the family into a lower social class, and the mother is more likely to enter the labor market in the areas of wholesale and retail, foodservice work, or accommodations,22, 23, 24, 25 which are usually physically strenuous and include long working hours, night shifts, and heavy lifting. Mothers who are involved in physically strenuous work are more likely to have LBW and PTB infants, which are associated with higher levels of child mortality.26

    This study found that LBW was associated with the highest child mortality, followed by PTB. In this study, the causes of death among PTB or LBW infants were bacterial sepsis of the newborn, unspecified (International Classification of Disease, ICD 10, P369), respiratory distress syndrome of the newborn (P220), Necrotizing Enterocolitis of the fetus and newborn (P77), congenital malformation of the heart, unspecified (Q249), and less than 28 weeks of gestation completed (P072), which are positively correlated with prematurity due to the short gestational age.

    The principal strength of this study is that it included national births from a 13-year period (1995–2007) in Korea, producing a nationwide retrospective cohort that constituted 99.43% of the total population. However, the findings of this study should be considered in light of its limitations. First, the national birth registration data, with which we employed in this study, have some omissions of cases of stillborn or neonatal deaths in the process of self-reporting of new births to the National Statistics Office, as the birth's parents might be reluctant to report the death cases to the national birth registration system.34, 35, 36, 37, 38 Second, misclassification could have occurred, thus underestimating the actual mortality levels. Third, parental education level and employment status may not accurately represent real social class differentials. These limitations could have underestimated the actual mortality levels and non-differential or differential misclassification might have occurred.

    In conclusion, this study showed that social class intensified the relationship between adverse birth outcomes and child mortality. The differences in child mortality between normal births and adverse births were greatest for parents with lower education (middle school of education or lower) and when the fathers were economically inactive and the mothers were in manual employment. Child mortality for adverse births was predominantly influenced by one parent in a lower social class, regardless of the spouse's social class. A father's economic inactive status, as well as maternal manual work, increased the risk of child mortality.

    This study suggests that child mortality should be reduced by considering adverse birth outcomes among parents in lower social classes. The parental social class should be considered to prevent child death due to adverse birth outcomes. The widening social inequalities might not be reduced by only focusing on social welfare policies and social welfare services without considering class relationships in a society.

    SUPPLEMENTARY MATERIAL

    Supplementary Table 1

    Relationships between Parental Social Class and Adverse Birth Outcomes (LBW and PTB)

    Click here to view.(30K, pdf)

    Notes

    The authors have no potential conflicts of interest to disclose.

    AUTHOR CONTRIBUTIONS:

    • Conceptualization: Mia Son.

    • Data curation: Mia Son and Soo-Jeong An.

    • Formal analysis: Mia Son, Soo-Jeong An, and Seung-Ah Choe.

    • Funding acquisition: Young-Ju Kim and Mijin Park.

    • Investigation: Mia Son, Seung-Ah Choe, and Soo-Jeong An.

    • Methodology: Mia Son, Seung-Ah Choe, and Soo-Jeong An.

    • Project administration: Young-Ju Kim and Mijin Park.

    • Resources: Young-Ju Kim and Mijin Park.

    • Software: Seung-Ah Choe and Soo-Jeong An.

    • Supervision: Young-Ju Kim and Mijin Park.

    • Validation: Young-Ju Kim and Mijin Park.

    • Visualization: Soo-Jeong An and Seung-Ah Choe.

    • Writing—original draft: Mia Son.

    • Writing—review & editing: Young-Ju Kim and Mijin Park.

    • Approval of final manuscript: all authors.

    ACKNOWLEDGEMENTS

    This research was supported by a grant of the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by the Ministry of Health & Welfare, Republic of Korea (grant number : HI19C1320).

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    MeSH Terms
    Child Mortality*
    Social Class*
    Adult
    Child
    Female
    Humans
    Infant
    Infant, Low Birth Weight
    Infant, Newborn
    Kaplan-Meier Estimate
    Male
    Pregnancy
    Pregnancy Outcome/epidemiology
    Premature Birth/epidemiology*
    Proportional Hazards Models
    Republic of Korea/epidemiology
    Retrospective Studies
    Socioeconomic Factors
    Metrics
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