1. Introduction
Preterm delivery is one of the main problems in the area of fertility [1]. The rate of preterm delivery in developed countries is between 5% and 15%, but in some places, it has been reported to range from 4% in the highest social classes to approximately 6% in the lowest social classes [2]. In 2005, the Institute of Medicine (IOM) estimated that the annual social cost of preterm delivery in the United States was $ 26 billion dollars, which included the cost of medical care for children under five years old, preterm delivery, maternity costs, and early intervention costs [3]. The rate of preterm delivery in Iran has been reported to be between 5.6 and 34.9%, which is the cause of 75 to 90% of neonatal mortalities [4].
Recent advances in the area of premature newborn care have increased the survival of these newborns. However, the increased prevalence of medical disabilities, learning difficulties, and behavioral and psychological problems among premature newborns have led to the concerns that these newborns may have more problems in their adulthood [5]. These problems include respiratory distress syndrome, dysplasia, anemia, chronic fatigue syndrome, dizziness, internal cerebral and abdominal bleeding, bacterial or fungal sepsis, retinopathy, necrotizing enterocolitis, learning and behavioral problems, mental retardation, blindness, hearing loss, and growth problems [6-9]. However, the significant growth of preterm delivery in different societies can cause numerous consequences, the most important of which is newborn mortality, which may lead to significant social and economic costs and consequences. There are specific causes for about half of preterm deliveries, but the exact cause is still unknown for the other half. Various factors play a role in this regard, and it is of great significance to identify the hidden communication channels between them [10]. 
Social determinants of health refer to the conditions, in which people are born, grow, live, and work. These conditions are among the key factors of equality in the area of health [11, 12]. Given the fact that living and working conditions are the determinants of most existing injustices, justice policies should focus on improving these living and working conditions and try to reduce the injustices [13]. Social justice is a matter of life and death. Socio-economic policies have a determinant effect on whether a child can grow and develop to their full potential and have a prosperous life or will have a ruined life [14]. Recently, different models have been proposed for illustrating the mechanisms of social determinants of health. One of the most complete models is the World Health Organization (WHO) Social Determinants of Health Model [15]. This model focuses on two main groups of determinants, namely, structural and intermediary determinants. Structural determinants refer to the factors, which create a social class, such as gender, income, education, and ethnicity, and intermediary factors include environmental conditions (e.g., work and housing), psycho-social conditions (e.g., psychosocial stresses), and behavioral factors (e.g., smoking) [16].
Objective
Given the global significance of preterm delivery and its short-term and long-term consequences for the infant, family, and society, and considering the uncertainty of its causes in 50% of cases and the lack of attention of public and researchers on this important issue, the identification of effective factors is necessary in this regard. Accordingly, the present systematic review was conducted to investigate the relationship between preterm delivery and socio-structural determinants of health with emphasis on occupation, education, and income in Iranian society based on the WHO model. 
2. Methods
Data sources
This study was a systematic review of all observational articles conducted in Iran on the relationship between preterm delivery and socio-structural determinants of health with emphasis on occupation, education, and income in Iranian society. The preferred reporting items for systematic reviews and meta-analyses (PRISMA) was used to identify and articulate the problem explored in this work, collect and analyze data, interpret the findings, and draw conclusions. Documents published from 2000 to November 2021 in international databases of Scopus, PubMed, Google Scholar, Web of Science, Embase, ProQuest, and Cochrane and the Persian databases of IranDoc and SID were reviewed. Keywords were extracted through Medical Subject Headings (MESH) terminology. The articles were searched using English and Persian keywords of occupation, education, income, social and economic status and preterm delivery, and appropriate operators of AND, OR, as well as a combination of the search strategy of each database. The search was performed by two researchers separately. All related articles in Iranian society were searched and collected. All unrelated or duplicate studies were excluded from the study.
Study Selection
Inclusion criteria included Studies on the relationship between preterm delivery (weeks 20 to 37 of pregnancy) and socio-structural determinants of health with emphasis on occupation, education, and income, studies conducted in Iran, observational studies, and those with available full text were included.
The following studies were excluded from the analysis: 
• Case studies
• Systematic review
• Chronic disease
• High-risk pregnancy
• Research on mental syndromes
• Original accessible articles 
• Unrelated reports
Examining the quality of studies 
The criterion of Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) was used for the evaluation of the quality of the articles, which was conducted by two researchers separately. In this 22-item checklist, the minimum and maximum scores of the studies range from 1 to 44. Based on the obtained scores, the articles were divided into three groups of low-quality (0-21), medium-quality (22-33), and high-quality (34-44) articles [17], and the Kappa coefficient was used to examine the agreement among the researchers. 
Data extraction
The search was conducted independently by two researchers in order to avoid bias. Information, such as the name of researchers, year of research, place of research, sample size, type of study, and structural determinants were extracted by two researchers separately.
3. Results 
Finally, after reviewing the title, inclusion criteria, abstract, and the main text of the articles, 17 articles were included in the study. All studies examined at least one of the structural determinants (occupation, education, income, and socio-economic status) and some of them investigated several structural determinants together (Figure 1).



According to the STROBE checklist, the studies were of medium and high quality, and the kappa agreement coefficient was K=0.79 among the researchers. Among the studies, 12 studies examined occupation, ten studies education, four studies income, and two studies socio-economic status (Table 1). Generally, seven studies on occupation, eight studies on education, and all studies on income and socio-economic status found a statistically significant relationship between these structural determinants and preterm delivery. The results showed that education, occupation and income, and economic and socio-economic status are associated with preterm delivery in Iran. 



4. Discussion 
Pregnancy and childbirth are considered special events for women and families. During this time, extensive physiological changes occur, which affect the body and soul of mothers. If a transition occurs from a physiological state to a pathological state, the health of the mother and neonate will be threatened. Moreover, a major goal for the socio-economic development of any society is to ensure the life of the mother and neonate and, hence, the role of the environment and its resources is very important and effective in this regard [32]. The health of mothers and newborns is basic to families and societies and of particular importance in ensuring and maintaining the health of the family and society [33]. Socio-economic status is evaluated at three levels of individual, family, and local. Although each level may seem independent, these levels affect each other’s consequences and can be examined at different stages of life [34]. The Commission on Social Determinants of Health (CSDH) framework assumes that structural factors produce or strengthen social stratification in society, which is used as a definition of people’s individual, social, and economic status and refers to the factors that lead to health injustice [35]. Health-related injustice within and among members of society is rooted in the adverse experiences of this condition in the early stages of life and can affect children’s brain development. There is an increased risk in this area and interventions seem necessary to reduce this risk [36].
Economic problems are one of the most important obstacles to the achievement of women to their health needs, especially during pregnancy and childbirth [37]. Because of poverty, inadequate pregnancy-related knowledge, and no awareness of the existence of service centers for periodic pregnancy examinations, women do not refer to these centers; there is also a strong relationship between education and one’s desire for receiving pregnancy care [38]. Peacock et al. (1995) argued that socioeconomic status can lead to preterm delivery in various ways, as preterm delivery is correlated with life events, marital status, income, housing, and education. The prevalence of health symptoms and problems during pregnancy, such as preterm delivery, is higher among the lower social classes; moreover, under the influence of stress, catecholamine is released and some changes are made in the concentration of other hormones [39]. Niedhammer et al. (2011) also maintained that the level of socioeconomic status, through the level of the mother’s education, is a significant predictor of preterm delivery [40].
As aware and ever-present members of the family, mothers provide their children with love and affection to improve their personal, social, and mental health [41]. Family education has a strong intergenerational effect and is considered a strong determinant of children’s health, survival, and education [42, 43]. Early life social injustice through childhood development and educational achievement leads to health injustice leading to low income and higher fertility in the future [44]. El-Sayed et al. (2012) examined the relationship between social factors, health indicators, and changes happening over time from 1989 to 2006; they found that over time, preterm delivery increased in more educated women, whereas it remained unchanged in less educated ones. Additionally, the risk of preterm delivery was lower in less educated women than the more educated ones [45]. In the study by Mortensen et al. (2009), education and income were related to preterm delivery [46]. In the study conducted by Agour et al. (2012), there was a relationship between education and preterm delivery [47]. Mothers’ education is a critical factor in newborn health, which can increase children’s chances of having a healthy and active life in adulthood [48]. Morgan et al. (2008) indicated that among five socio-economic variables (education of husband and wife, occupation of husband and wife, and income), the level of mother’s education was a strong predictor of preterm delivery. Moreover, in this study, occupation and low income were not risk factors for preterm delivery [49].
Some explain the relationship between education and health through occupation and economic conditions, meaning that the higher the education, the more likely you are to be employed in healthier, higher-paying jobs and workplaces that allow you to have more control over your life. These conditions, thus, contribute to the health of family members. More financial resources lead to the adoption of a healthier lifestyle and better nutrition as well as access to better health services [15]. As one of the most important economic categories, income includes various aspects, such as knowledge and physical and mental health. Income inequality is one of the unpleasant phenomena of social life, and its reduction and elimination are a top priority in almost all human societies [50]. As a complicated variable, income has a cumulative effect on life, which can often change in short-term periods [51]. Income is, thus, an approximate indicator of access to resources and living standards [52, 53]. Joseph et al. (2014) revealed that preterm delivery was spontaneously correlated with income [54]. Bibby et al. (2004) considered low social class, low education level, and low income as the risk factors for preterm delivery [55]. Moreover, poverty can lead to social segregation and isolation. Societies with high levels of income inequality are less likely to enjoy social cohesion and, consequently, suffer from higher levels of violence [12].
Employment promotes the growth and value of women, because social interactions strengthen their communication and decision-making ability and their access to resources and, generally, improve their social status [55]. Social consequences, authority, and independence are among the positive aspects of women’s employment, interference with household plans and parenting is mentioned as the negative aspect of their employment [56]. Working conditions can also affect the health and justice of individuals and society. Poor working conditions can affect mental health almost as much as job loss. Poor working conditions, such as prolonged standing and exposure to chemicals can develop adverse consequences, such as miscarriage, preterm delivery, low birth weight, and birth defects. Job stress can also impose negative effects on fetal growth and development [27].
5. Conclusion 
Preterm delivery is a prevalent problem with critical complications in Iran and structural determinants have a significant relationship with preterm delivery. Proper interventions, such as life skills training, self-care, and prenatal care can contribute to the improvement of pregnancy outcomes. Most studies in Iran have investigated demographic factors and no study has examined the impact of economic inequality on the incidence of preterm delivery. Future studies can investigate the effect of inequality on the incidence of preterm delivery. 
Ethical Considerations
Compliance with ethical guidelines
This study was approved by the Ethics Committee of the University of shahid beheshti university of medical scinces (Code: IR.SBMU.PHARMACY.REC.1400.276). 
Funding
This study was supported by the Midwifery and Reproductive Health Research Center, Department of Midwifery and Reproductive Health, School of Nursing and Midwifery, Shahid Beheshti University of Medical Sciences, Tehran (Code: IR.SBMU.PHARMACY.REC.1400.276).
Authors' contributions
All authors equally contributed to preparing this article.
Conflicts of interest
The authors declared no conflict of interest.
Acknowledgments
The authors express their gratitude to the researchers whose articles were analyzed and the Nursing and Midwifery Faculty of the University of Medical Sciences for the support of this work.


References