The Decomposition of Socioeconomic Factors in Inequality in Preterm Birth; the Results of a Cohort Study Based on the Concentration Index Approach

Preterm labor is a crucial factor in neonatal morbidity and mortality. The present study aims to investigate the effects of Socioeconomic Status (SES) on preterm birth and to decompose the main inequalities affecting preterm birth.

In a cohort study in 2017, 458 pregnant women in Ilam, Iran, were followed from the 20th gestational week up to the labor. All participants were followed up according to the Prenatal Care Schedule approved by the Ministry of Health of the Islamic Republic of Iran from the beginning to the end of the study. The results of participants' laboratory tests were collected based on the records from their pregnancy documents. A code was inserted in the prenatal care booklet of each participant for exchanging information between the researcher and participants. The Concentration Index (CI) was used to determine inequality. The Inequality Index was decomposed to find out the main sources of inequality in preterm infants. Distributive Analysis Stata Package was used for estimating CI.

Of the total of 458 participants, 12% had preterm labor. The number of male newborns was 219 (47.8%). Also, the prevalence of preterm labor was more in mothers aged 20 to 31 years (p < 0.001), housewives (p=0.037), and women with lower socioeconomic status (p < 0.001). The socioeconomic contributors to inequality were the mother’s age (p=0.008), father’s age (p=0.005), father’s education (p=0.003), and father's job (p < 0.001). The main factors of inequality were mother’s age (8%), father’s age (3%), father’s education (5%), and father's job (25%).

Overall, 12% of our study population had preterm labor. Preterm birth was significantly more observed among mothers of lower socioeconomic status. Some SES, including the mother's age, father’s age, father’s education, and father’s job, were important factors in preterm inequality among Iranian mothers.