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Economic perturbations and fetal growth: A multilevel analysis of exposure to labor market insecurity during gestation and birth weight for gestational age

  • Author(s): Margerison-Zilko, Claire E.
  • Advisor(s): Ahern, Jennifer
  • et al.
Abstract

Economic perturbations and fetal growth: A multilevel analysis of exposure to labor market insecurity during gestation and birth weight for gestational age

by

Claire E. Margerison-Zilko

Doctor of Philosophy in Epidemiology

University of California, Berkeley

Professor Jennifer Ahern, Chair

Background. Epidemiologic research has made important strides in identifying individual-level risk factors for adverse birth outcomes such as low birth weight and preterm birth, which carry high clinical, social, and economic costs. Despite this accumulated knowledge, we remain unable to explain differences in the distribution of birth outcomes between populations and within populations across space and time, suggesting the need to consider macro-level, ecologic determinants of birth outcomes. In this dissertation, I developed a conceptual model based on ecologic and evolutionary theory and research that proposes that unexpected changes to the human ecology, i.e. perturbations, may result in unexpected behavioral and biological responses in humans and that such responses will be conserved by natural selection if they are adaptive. Based on this framework, I hypothesized that reductions in fetal growth would occur in response to maternal exposure to ecological perturbations--specifically, perturbations to labor market security--during gestation.

Methods. I examined the association between maternal exposure to state-level labor market perturbations during each trimester of gestation and fetal growth, as measured by birth weight for gestational age percentile. The study population included 6,715 gestations and births between 1982 and 2000 to women enrolled in the National Longitudinal Survey of Youth 1979 (NLSY79). I calculated birth weight for gestational age percentiles using national reference data and categorized births <10th percentile as small for gestational age (SGA).

I defined perturbations to labor market security as months in which the state unemployment rate was higher than its statistically expected value (i.e., unexpectedly high labor market insecurity) and months in which the state unemployment rate was lower than its statistically expected value (i.e., unexpectedly high security). I derived statistically expected values using ARIMA modeling methods to account for autocorrelation. Gestations in the NLSY79 were classified as either exposed or unexposed to labor market insecurity or security in the first, second, and third trimester if one of these labor market perturbations occurred in the maternal state of residence during that trimester.

I used linear and logistic regression models to examine the association between labor market perturbations in each trimester and birth weight percentile and odds of SGA. I also examined whether any observed associations differed by maternal race/ethnicity, childhood socioeconomic status, educational attainment, marital status, employment status, or poverty status. Finally, I explored whether any observed associations were mediated by individual economic change (i.e., changes in maternal employment status or household income) or maternal pregnancy behaviors (i.e., smoking, first trimester utilization of prenatal care, or net gestational weight gain). If associations were mediated by one these factors, I calculated the proportion of the total association explained by that factor.

Results. Exposure to labor market insecurity in the first trimester was significantly associated with a decrease in birth weight for gestational age of 4.05 percentile points (95% CI = -6.87, -1.22) and higher odds of SGA (OR = 1.50, 95% CI = 1.21, 1.86). Exposure to labor market insecurity in the second and third trimesters was not significantly associated with either outcome. Exposure to labor market security was not associated with birth weight for gestational age percentile or SGA.

The association between exposure to labor market insecurity in the first trimester and birth weight percentile differed significantly by maternal childhood SES, educational attainment, and employment status but not by race/ethnicity, marital status, or poverty status. Exposure to labor market insecurity in the first trimester was associated with decreases in birth weight percentile of 5.52 (95% CI = -10.0, -1.04) and 8.66 points (95% CI = -14.04, -3.29) among women with average or high childhood SES, respectively, while the association was not significant among women with low childhood SES. Exposure to labor market insecurity in the first trimester was associated with a decrease in birth weight percentile of 9.22 points (95% CI = -15.77, -2.88) among women with <12 years educational attainment, while the association was not significant among women with 12 years or >12 years educational attainment. Exposure to labor market insecurity was associated with a decrease in birth weight percentile of 7.10 points (95% CI = -12.33, -1.87) and 10.27 points (95% CI = (-18.82, -1.71) among women keeping house and out of the labor force, respectively, while the association was not significant among employed and unemployed women.

My exploration of mediation by individual economic change and maternal pregnancy behaviors found that approximately 11% of the association between exposure to labor market insecurity in the first trimester and birth weight percentile was explained by net maternal gestational weight gain. The association also differed significantly by maternal smoking, with the association only significant among smokers. No other individual economic change or maternal pregnancy behaviors mediated greater than one percent of the association.

Conclusions. Findings support my hypothesis that fetal growth responds to a contemporary ecological perturbation, i.e., unexpectedly high labor market insecurity. Exposure to this perturbation appears to have more impact on fetal growth if it occurs in the first trimester of gestation. The finding that associations between exposure to labor market insecurity and birth weight percentile were stronger among women with high childhood SES, <12 years education, and those keeping house or out of the labor force suggests that these women may be more vulnerable to economic perturbations. Although further research on mediation is needed, initial findings suggest that maternal gestational weight gain may represent one pathway through which economic perturbations affect fetal growth.

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