Abstract

The incidence of premature birth has increased worldwide, unequally distributed by race/ethnicity. Racism generates economic inequalities, educational disparities, and differential access to health care, which increases the risk of preterm birth. Thus, this study aimed to evaluate the factors associated with preterm birth and racial and ethnic disparities in premature birth among pregnant women attending prenatal care at the Brazilian Unified Health System health units in the urban area of Santo Antônio de Jesus, Bahia, Brazil. This study used data from 938 pregnant women aged between 18 to 45 years within the NISAMI prospective cohort. Premature birth prevalence was 11.8%, with a higher prevalence among black than non-black women (12.9% versus 6.0%, respectively). Maternal age between 18 and 24 years was the only factor associated with premature birth. A higher risk of premature birth was found among black women than non-black women (RR 3.22; 95%CI 1.42-7.32). These results reveal the existence of racial and social inequalities in the occurrence of premature birth.

Key words:
Premature; Health inequalities; Ethnicity and health; Cross-sectional studies; Prevalence

Resumo

A incidência de parto prematuro tem aumentado em todo o mundo, distribuída de forma desigual por raça/etnia. O racismo gera desigualdades econômicas, disparidades educacionais e acesso diferenciado à saúde, o que aumenta o risco de parto prematuro. Assim, este estudo teve como objetivo avaliar os fatores associados à prematuridade e disparidades raciais e étnicas no parto prematuro entre gestantes atendidas durante o pré-natal em unidades de saúde do Sistema Único de Saúde na zona urbana de Santo Antônio de Jesus, Bahia, Brasil. Este estudo utilizou dados de 938 mulheres grávidas com idade entre 18 e 45 anos dentro da coorte prospectiva do NISAMI. A prevalência de prematuridade foi de 11,8%, sendo maior entre as negras do que entre as não negras (12,9% versus 6,0%, respectivamente). A idade materna entre 18 e 24 anos foi o único fator associado ao parto prematuro. Foi encontrado maior risco de prematuridade entre as mulheres negras do que entre as não negras (RR 3,22; IC95% 1,42-7,32). Esses resultados revelam a existência de desigualdades raciais e sociais na ocorrência do parto prematuro.

Palavras-chave:
Prematuro; Desigualdades em saúde; Etnicidade e saúde; Estudos transversais; Prevalência

Introduction

Premature birth is defined as birth occurring before the 37^th week of pregnancy and represents a major cause of infant morbidity and mortality globally¹1 Stella CL, Bennett MR, Devarajan P, Greis K, Wyder M, Macha S, Rao M, Jodicke C, Moussa H, How HY, Myatt L, Webster R, Sibai BM. Preterm labor biomarker discovery in serum using 3 proteomic profiling methodologies. Am J Obstet Gynecol 2009; 201(4):387.e1-387.e13.

2 Callahan BJ, DiGiulio DB, Aliaga Goltsman DS, Sun CL, Costello EK, Jeganathan P, Biggio JR, Wong RJ, Druzin ML, Shaw GM, Stevenson DK, Holmes SP, Relman DA. Replication and refinement of a vaginal microbial signature of preterm birth in two racially distinct cohorts of US women. Proc Natl Acad Sci USA 2017; 114(37):9966-9971.^-33 Dória MT, Spautz CC. Trabalho de parto prematuro predição e prevenção. Femina 2011; 39(9):443-449. and a significant contributor to childhood morbidities. Preterm infants are particularly vulnerable to complications due to impaired respiration, difficulty feeding, poor body temperature regulation, and a high risk of infection⁴4 World Health Organization (WHO). Recommendations on interventions to improve preterm birth outcomes. Geneva: WHO; 2019.. In addition, children born too soon may face a lifetime disability, including learning disabilities and visual and hearing problems⁵5 Howson CP, Kinney MV, Lawn J. World Health Organization. Born Too Soon: the global action report on preterm birth. Geneva: WHO; 2012..

A systematic review published in 2010, which aimed to analyze the rates and map the distribution of premature births globally, estimated that about 12.9 million births, or 9.6% of all births, were preterm. Of these, 85% were concentrated in Africa and Asia (10.9 million premature births), while in Europe and North America, there were 0.5 million premature births, and 0.9 million occurred in Latin America and the Caribbean⁶6 Beck S, Wojdyla D, Say L, Betran AP, Merialdi M, Requejo JH, Rubens C, Menon R, Van Look PFA. The worldwide incidence of preterm birth: a systematic review of maternal mortality and morbidity. Bull World Health Organ 2010; 88(1):31..

Between 2013 and 2018, almost 2 million (11.1%) Brazilian babies were premature⁷7 Alberton M, Rosa, VM, Ise BPM. Prevalência e tendência temporal da prematuridade no Brasil antes e durante a pandemia de covid-19: análise da série histórica 2011-2021. Epidemiol Serv Saude 2023; 32(2):14., which is very close to the prevalence reported in countries such as Indonesia (10.4%), Nigeria (11.4%), and Ethiopia (12.0%)⁸8 Chawanpaiboon S, Vogel JP, Moller AB, Lumbiganon P, Petzold M, Hogan D, Landoulsi S, Jampathong N, Kongwattanakul K, Laopaiboon M, Lewis C, Rattanakanokchai S, Teng DN, Thinkhamrop J, Watananirun K, Zhang J, Zhou W, Gülmezoglu AM. Global, regional, and national estimates of levels of preterm birth in 2014: a systematic review and modelling analysis. Lancet Glob Health 2019; 7(1):e37-e46.. Also, studies have demonstrated an increase in the rate of premature births over time in Brazil; in the state of Bahia, the prevalence of premature births increased from 10.9% in 2000 to 11.4% in 2011⁹9 Matijasevich A, Silveira MF, Matos ACG, Rabello Neto D, Fernandes RM, Maranhão AG, Cortez-Escalante JJ, Barros FC, Victora CG. Estimativas corrigidas da prevalência de nascimentos pré-termo no Brasil, 2000 a 2011. Epidemiol Serv Saude 2013; 22(4):557-564., and in the Rio Grande do Sul, preterm births increased from 5.8% in 1982 to 13.8% in 2015¹⁰10 Silveira MF, Victora CG, Horta BL, Silva BGC, Matijasevich A, Barros FC, Barros AJD, Menezes AMB, Bertoldi AD, Bassani DG, Wehrmeister FC, Gonçalves H, Santos IS, Murray J, Tovo-Rodrigues L, Assunção MCF, Domingues MR, Hallal PRC. Low birthweight and preterm birth: trends and inequalities in four population-based birth cohorts in Pelotas, Brazil, 1982-2015. Int J Epidemiol 2019; 48(Supl.1):i46-i53..

Preterm births cause a high social and financial cost to family members and society and often require infrastructure and highly technical staff, which are not always available. The causes of premature births are multifactorial¹¹11 Silveira MF, Santos IS, Matijasevich A, Malta DC, Duarte EC. Nascimentos pré-termo no Brasil entre 1994 e 2005 conforme o Sistema de Informações sobre Nascidos Vivos (SINASC). Cad Saude Publica 2009; 25(6):1267-1275. and can be related to the prenatal period and the delivery. Research has identified risk factors for premature birth, such as race/ethnicity, maternal age, education, socioeconomic conditions, smoking, maternal employment, nutritional state, and others¹²12 Vogel JP, Chawanpaiboon S, Moller AB, Watananirun K, Bonet M, Lumbiganon P. The global epidemiology of preterm birth. Best Pract Res Clin Obstet Gynaecol 2018; 52:3-12..

There are evidence that racial/ethnic disparities can cause premature birth. A cohort study conducted in the United States showed that premature births among black women occurred independently of medical and maternal socioeconomic factors; additionally, a study of the epidemiology and causes of premature birth showed that women categorized as black or Afro-descendant had a higher risk of preterm delivery¹³13 Goldenberg RL, Culhane JF, Iams JD, Romero R. Epidemiology and causes of preterm birth. Lancet 2008; 371(9606):75-84.^,1414 Kistka ZAF, Palomar L, Lee KA, Boslaugh SE, Wangler MF, Cole FS, DeBaun MR, Muglia LJ. Racial disparity in the frequency of recurrence of preterm birth. Am J Obstet Gynecol 2007; 196(2):131.e1-131.e6.. Furthermore, a systematic review and meta-analysis found an odds ratio of preterm birth of 1.99 (95%CI 1.83-2.16) among black women, whereas no significant associations were observed among Asian, Hispanic, and Caucasian women¹⁵15 Schaaf J, Liem S, Mol B, Abu-Hanna A, Ravelli A. Ethnic and Racial Disparities in the Risk of Preterm Birth: A Systematic Review and Meta-Analysis. Am J Perinatol 2012; 30(6):433-450..

Brazil also has a disparity in premature birth, which is related to race/ethnicity. The prevalence was higher in pregnant women indigenous race/skin color (14.4%), when compared to other ethnicities. Black pregnant women had a higher prevalence compared to white and brown categories⁷7 Alberton M, Rosa, VM, Ise BPM. Prevalência e tendência temporal da prematuridade no Brasil antes e durante a pandemia de covid-19: análise da série histórica 2011-2021. Epidemiol Serv Saude 2023; 32(2):14..

Structural racism produces practices, beliefs, behaviors, and prejudices that favor avoidable and unfair inequalities between social groups by obstructing access to goods, resources, services, and opportunities¹⁶16 Goes EF, Ramos DO, Ferreira AJF, Goes EF, Ramos DO, Ferreira AJF. Desigualdades raciais em saúde e a pandemia da Covid-19. Trab Educ Saude 2020; 18(3):e00278110., enforcing forms of discrimination such as neighborhood deprivation, economic inequalities, educational disparities, and differential access to health care. Consistently, these factors increase the risk for preterm birth and infant mortality¹⁷17 Beck AF, Edwards EM, Horbar JD, Howell EA, McCormick MC, Pursley DM. The color of health: how racism, segregation, and inequality affect the health and well-being of preterm infants and their families. Pediatr Res 2020; 87(2):227-234.. However, the association between racial/ethnic disparities and premature birth is still not completely understood¹⁸18 Lu MC, Halfon N. Racial and Ethnic Disparities in Birth Outcomes: A Life-Course Perspective. Maternal Child Health J 2003; 7(1):13-30.

19 Gage TB, Fang F, O'Neill EK, DiRienzo AG. Racial disparities in infant mortality: What has birth weight got to do with it and how large is it? BMC Pregnancy Childbirth 2010; 10(1):1-14.^-2020 Swamy GK, Edwards S, Gelfand A, James SA, Miranda ML. Maternal age, birth order, and race: differential effects on birthweight. J Epidemiol Community Health (1978). 2012; 66(2):136.. Thus, there is a need for more comprehensive studies on maternal influences and racial/ethnic disparities in gestational outcomes²¹21 Mason LR, Nam Y, Kim Y. Validity of Infant Race/Ethnicity from Birth Certificates in the Context of U.S. Demographic Change. Health Serv Res 2014; 49(1):249.^,2222 Straughen JK, Caldwell CH, Young AA, Misra DP. Partner support in a cohort of African American families and its influence on pregnancy outcomes and prenatal health behaviors. BMC Pregnancy Childbirth 2013; 13(1):1-9., especially in South American countries, where direct research on this question is sparse.

In this context, the current study aimed to evaluate the factors associated with preterm birth and racial and ethnic disparities in premature birth among pregnant women attending prenatal care at the Brazilian Unified Health System in the urban area of Santo Antônio de Jesus, Bahia, Brazil.

Methods

This study used data from the prospective cohort “Maternal risk factors of low birthweight, premature birth, and intrauterine growth restriction in the Recôncavo of Bahia”, conducted by the Maternal and Child Health Research Center (NISAMI) of the Center of Health Sciences at the Federal University of the Recôncavo of Bahia. The population of this study consisted of pregnant women receiving prenatal care at the Unified Health System (SUS) basic health units in the urban area of Santo Antônio de Jesus, Bahia, Brazil, between 2011 and 2015.

The city of Santo Antônio de Jesus is in the Recôncavo of Bahia, 187 km from the capital city of Salvador²³23 Instituto Brasileiro de Geografia e Estatística (IBGE). Cidades: Santo Antônio de Jesus [Internet]. [acessado 2016 mar 20]. Disponível em: https://cidades.ibge.gov.br/brasil/ba/santo-antonio-de-jesus/panorama.
https://cidades.ibge.gov.br/brasil/ba/sa... . The average number of live births from 2003-2012 was 1,371.6 per year, and 5.26% were premature²⁴24 Brasil. Departamento de Informática do Sistema Único de Saúde do Brasil (DATASUS). Sistema de informação sobre Nascidos Vivos [Internet]. [acessado 2014 nov 25]. Disponível em: http://tabnet.datasus.gov.br/cgi/tabcgi.exe?cnes/cnv/estabba.def.
http://tabnet.datasus.gov.br/cgi/tabcgi.... . The provision of health services took place in 26 primary care units (18 in the urban area and 8 in the rural area), 38 clinics/specialty centers, five hospitals, and two polyclinics.

This study was conducted across all the SUS’s primary care clinics (basic health units) in the urban area of the municipality. The primary care clinics of the rural area were excluded due to difficulties in access, as well as women with multiple pregnancies (twins or triplets). Thus, all pregnant women aged 18 and older residing and domiciled in the urban area of the municipality - regardless of gestational age - registered in the Monitoring System of Humanization of Prenatal and Births (SISPRENATAL) and who had attended at least one prenatal visit participated in this study.

Two sources of information were used for data collection: a survey given to puerperal women and the registry of live births. The first data source assessed information related to the independent variable and covariates using a structured interview. Data relating to premature births were analyzed from the second data source. All the study instruments were reviewed and tested by a team of supervisors. Validation was conducted by comparing data obtained by the survey in relation to data registered on the prenatal cards. Field supervisors revisited twenty percent of the pregnant women interviewed and partially reapplied the interview. The data collected were then compared with the original interview to evaluate quality, aiming to identify any imprecision, systematic error, or fraud.

The program OpenEpi was used to determine the study’s sample size, based on a frequency of premature birth of 7.83 among those not exposed and a relative risk of 2.27¹⁴14 Kistka ZAF, Palomar L, Lee KA, Boslaugh SE, Wangler MF, Cole FS, DeBaun MR, Muglia LJ. Racial disparity in the frequency of recurrence of preterm birth. Am J Obstet Gynecol 2007; 196(2):131.e1-131.e6.. The following parameters were also used: 80% power, a significance level of 5%, and adding 20% for loss to follow-up. The required sample size for this study was therefore calculated as 938 women.

The dependent variable was defined as the dichotomous variable of premature birth, evaluated using the definition adopted by the WHO (1961) of delivery before the 37^th week of gestation²⁵25 Almeida TSO, Lins RP, Camêlo AL, Mello DCCL. Investigação sobre os fatores de risco da prematuridade: uma revisão sistemática. Rev Bras Cien Saude 2013; 17(3):301-308.. All infants born whose gestation was less than 37 weeks were considered preterm, and the reference group was those live births whose gestational age was equal to or greater than 37 weeks according to the date of last menstruation.

Race/ethnicity was the independent variable and was assessed in the following manner: “In your opinion, how do you define your race/ethnicity?”. This information was collected by self-assessment based on the categories used by the Brazilian census (IBGE): white, black, brown (mixed race), Asian, and Brazilian indigenous. For this analysis, the variable was dichotomized into black and non-black women, where the category of black included all women who self-identified as black or brown. The variable race/ethnicity was analyzed as a social-historical construct.

The exposure covariates were defined using risk factors for premature birth, including socioeconomic variables, lifestyle, and obstetric history.

The program EPI-DATA version 3.0 was used to enter data, and Stata version 12.0 was used to conduct the statistical analysis. For the analysis, the population was first characterized according to the principal independent variable and the covariates of the study, using the Pearson Chi-square test (X²) and a p-value of ≤0.05 for the significant association.

The bivariate analysis was then conducted to assess the association between the covariates and the occurrence of premature birth, using the Risk Ratio (RR) as the outcome measure with a 95% confidence interval (95%CI) estimated through Poisson regression with robust error variance. First, variables with a p-value less than or equal to 0.20 in the crude analysis were included in the multivariate Poisson regression analysis. Next, these variables were introduced in the model using a backward stepwise procedure. Finally, variables that remained significant, those with a p-value≤0.05, were kept in the model.

The project “Maternal risk factors of low birthweight, premature birth, and intrauterine growth restriction in the Recôncavo of Bahia” was submitted and approved by the Ethics and Research Committee of the Faculdade Adventista de Fisioterapia da Bahia (FAFIS), under protocol number 4369.0.000.070-10 and opinion number 050/2010.

Results

From 2011 to 2015, 1,091 pregnant women met the initial study selection criteria. Overall, data from gestational age at delivery was available for 938 (86.0%) pregnant women within the NISAMI Cohort, which were included in the present study.

Concerning socioeconomic characteristics and obstetric history, this sample ranged in age from 18-45 years, with a mean of 25.8 years (SD±8.48); a higher proportion of women were aged 25-34 years (50.9%). Regarding socio-demographics, 43% of the women had a high school education, 47.6% had a household income of 2-4 times the minimum wage, and 83.1% lived with a partner. Cesarean section was the most common type of delivery (63.7%), non-induced labor was more frequent than induced labor (78.1%), and the majority of women (74.9%) began prenatal care in the first trimester of pregnancy (Table 1).

Regarding lifestyle, 60.1% of the women reported that they stopped using alcohol, 64.3% never smoked cigarettes, more than 95% never used other kinds of drugs, and over 90% reported that they did not participate in any kind of physical activity. Among the sample, 84% self-identified their race/ethnicity as black or brown. Only household income was statistically significantly associated when comparing socioeconomic characteristics, lifestyle, and obstetric history by maternal race/ethnicity.

Analyses showed a statistically significant positive association between the occurrence of premature birth and the age group of 18-24 years (RR1.72; 95%CI 1.18-2.50) (Table 2).

The prevalence of premature birth in this study was 11.8% (n=111; 95%CI 9.9-14.1%): 12.9% (n=102) among black and 6.0% (n=9) among non-black women. There was a statistically significant association between maternal race/ethnicity and premature birth in the crude analysis, revealing a 2.16 times higher risk of premature birth among black women compared to non-black women (95%CI 1.12-4.17) (Table 3).

In the multivariable analysis, newborns of the female sex, Cesarean section delivery, and the onset of prenatal care during the first trimester were no longer associated with premature birth according to maternal race/ethnicity. Additional analysis showed a higher risk of premature birth among women who had induced labor.

Maternal race/ethnicity maintained a positive association with premature birth even after controlling for covariates in the final model, in which black women have a 3.22 higher risk of premature birth than non-black women (95%CI 1.42-7.32) (Table 3).

Discussion

The results of this study show difference in premature birth by maternal race/ethnicity, where black women have almost three times the risk of premature birth than non-black women. These findings corroborate results from studies of the United States and the United Kingdom. A study using data from vital statistics on births to primiparous women in the State of Nebraska, in the United States, from 2005 to 2014 found that black women had a 1.33 times higher risk of premature birth than white women²⁶26 Su D, Samson K, Hanson C, Anderson Berry AL, Li Y, Shi L, Zhang D. Racial and ethnic disparities in birth Outcomes: A decomposition analysis of contributing factors. Prev Med Rep 2021; 23:101456.. A cross-sectional study also conducted in the United States evaluated racial/ethnic differences in preterm births in 2016 birth certificate data and concluded that premature birth has 1.46 times higher odds of occurring in black women than in white women²⁷27 Thoma ME, Drew LB, Hirai AH, Kim TY, Fenelon A, Shenassa ED. Black-White Disparities in Preterm Birth: Geographic, Social, and Health Determinants. Am J Prev Med 2019; 57(5):675-686.. Similarly, a population-based study using routinely collected and linked national data on all singleton live births in England and Wales between 2006 and 2012 observed higher risks of premature birth among the black Caribbean and African women compared to white British women²⁸28 Li Y, Quigley MA, Macfarlane A, Jayaweera H, Kurinczuk JJ, Hollowell J. Ethnic differences in singleton preterm birth in England and Wales, 2006-12: Analysis of national routinely collected data. Paediatr Perinat Epidemiol 2019; 33(6):449-458..

Regarding studies conducted in Brazil, a cohort study from the state of São Paulo, showed that race is an independent risk factor for premature birth, even after adjusting for household income and maternal education²⁹29 Silva LM, Silva RA, Silva AAM, Bettiol H, Barbieri MA. Racial inequalities and perinatal health in the southeast region of Brazil. Braz J Med Biol Res 2007; 40(9):1187-1194.. Racial differences in premature birth can be explained by the socioeconomic disadvantages experienced by black women since these women face greater social and economic challenges than white women³⁰30 Fonseca JM, Silva AAM, Rocha PRH, Batista RLF, Thomaz EBAF, Lamy-Filho F, Barbieri MA, Bettiol H. Racial inequality in perinatal outcomes in two Brazilian birth cohorts. Braz J Med Biol Res 2021; 54(1):1-9.^,3131 Guimarães JMN, Yamada G, Barber S, Caiaffa WT, Friche AAL, Menezes MC, Santos G, Santos I, Cardoso LO, Diez Roux AV. Racial Inequities in Self-Rated Health Across Brazilian Cities: Does Residential Segregation Play a Role? Am J Epidemiol 2022; 191(6):1071-1080.. Yet these differences can be influenced by other factors, such as difficulty in accessing prenatal care caused by institutional racism³²32 Domingues PML, Nascimento ER, Oliveira JF, Barral FE, Rodrigues QP, Santos CCC, Araújo EM. Discriminação racial no cuidado em saúde reprodutiva na percepção de mulheres. Texto Contexto Enferm 2013; 22(2):285-292.. Institutional racism is the weakness of institutions in providing adequate services to people by their race, culture, racial origin, or ethnicity, placing them in a disadvantageous situation in accessing benefits generated by the State or other organized institutions³³33 Kalckmann S, Santos CG, Batista LE, Cruz VM. Racismo institucional: um desafio para a eqüidade no SUS? Saude Soc 2007; 16(2):146-155..

A Brazilian cross-sectional study with 5,289 women that evaluated the influences of the race on adverse obstetric and neonatal outcomes found that most black women were young, possessed lower levels of education, and lived at the minimum wage compared to white women³⁴34 Pacheco VC, Silva JC, Mariussi AP, Lima MR, Silva TR. As influências da raça/cor nos desfechos obstétricos e neonatais desfavoráveis. Saude Debate 2018; 42(116):125-137.. Similar results were observed in our study, demonstrating that the maps of poverty can be superimposed on the distribution of race/ethnicity. In Brazil, black people occupy the less qualified and lower-compensated positions in the labor market, have lower levels of education, and live in areas that offer fewer services, less basic infrastructure, and suffer greater restrictions in access to healthcare services that, when received, are of worse quality and lower resolution³⁵35 Articulação para o Combate ao Racismo Institucional (CRI). Identificação e abordagem do racismo institucional. Brasília: CRI; 2006..

Maternal age was an important factor associated with premature birth in the present study, with a higher proportion of the outcome (15.4%) among women between 18 and 24 years. This finding differs from previous Brazilian studies, in which a higher prevalence of preterm birth was observed among pregnant adolescents³⁶36 Nunes FBB F, Silva PC, Barbosa TLSM, Lopes MLH, Silva EL. Influence of maternal age in perinatal conditions in live births of São Luís, Maranhão. Rev Pesq Cuidado Fundamental Online 2019; 12:292-299. and those aged 40 years or older³⁷37 Martinelli KG, Dias BAS, Lemos Leal M, Belotti L, Garcia ÉM, Santos Neto ET. Prematuridade no Brasil entre 2012 e 2019: dados do Sistema de Informações sobre Nascidos Vivos. Rev Bras Estud Popul 2021; 38:1-15..

Also, a population‐based cross‐sectional study using the California Office of Statewide Health Planning and Development linked birth cohort data from 2008 to 2012 found higher premature birth rates among women younger than 15 years and 40 years or older³⁸38 Keiser AM, Salinas YD, DeWan AT, Hawley NL, Donohue PK, Strobino DM. Risks of preterm birth among non-Hispanic black and non-Hispanic white women: Effect modification by maternal age. Paediatr Perinat Epidemiol 2019; 33(5):346-356.. Immaturity of the uterus or the blood supply of the cervix in teenage pregnancy can increase the risk of subclinical infection and production of prostaglandins, triggering an increased risk of preterm delivery. At the same time, in late pregnant women, prematurity is associated with urinary tract infection, chronic diseases, and pregnancy complications, which are more frequent in pregnant women over 40 years³⁹39 Fuchs F, Monet B, Ducruet T, Chaillet N, Audibert F. Effect of maternal age on the risk of preterm birth: A large cohort study. Gutman J, editor. PLoS One 2018; 13(1):e0191002.^,4040 Sass A, Gravena AAF, Pelloso SM, Marcon SS. Resultados perinatais nos extremos da vida reprodutiva e fatores associados ao baixo peso ao nascer. Rev Gaucha Enferm 2011; 32(2):362-368..

Social determinants of health, the non-medical factors that influence health outcomes, are the conditions in which people are born, grow, work, live, and age, which influence health inequities⁴¹41 World Health Organization (WHO). Social determinants of health [Internet]. 2022 [cited 2022 jul 12]. Available from: https://www.who.int/health-topics/social-determinants-of-health#tab=tab_1.
https://www.who.int/health-topics/social... . As stated before, racism enforces discrimination, generating educational disparities, economic inequalities, neighborhood deprivation, and differential health care access, which may increase the risk for premature birth¹⁷17 Beck AF, Edwards EM, Horbar JD, Howell EA, McCormick MC, Pursley DM. The color of health: how racism, segregation, and inequality affect the health and well-being of preterm infants and their families. Pediatr Res 2020; 87(2):227-234.. As a result, preterm birth disproportionately affects black and poor infants. Although healthcare quality and access improvements help decrease these disparities, they are not sufficient to eliminate them⁴²42 Burris HH, Hacker MR. Birth outcome racial disparities: A result of intersecting social and environmental factors. Semin Perinatol 2017; 41(6):360-366..

The socioeconomic position is reproducibly associated with an increased risk of preterm birth, and higher income is associated with improving this outcome⁴²42 Burris HH, Hacker MR. Birth outcome racial disparities: A result of intersecting social and environmental factors. Semin Perinatol 2017; 41(6):360-366.. In our study, household income was not associated with maternal race/ethnicity. This contrasts with findings from a previous Brazilian cohort study, where the authors observed that women with lower income are at greater risk of having preterm infants than women with higher income⁴³43 Sadovsky ADI, Matijasevich A, Santos IS, Barros FC, Miranda AE, Silveira MF. Socioeconomic inequality in preterm birth in four Brazilian birth cohort studies. J Pediatr (Rio J) 2018; 94(1):15-22.. Additionally, lack of or low income may influence access to services, nutrition, and emotional issues, and increase stress during pregnancy⁴³43 Sadovsky ADI, Matijasevich A, Santos IS, Barros FC, Miranda AE, Silveira MF. Socioeconomic inequality in preterm birth in four Brazilian birth cohort studies. J Pediatr (Rio J) 2018; 94(1):15-22., which is associated with premature birth¹⁷17 Beck AF, Edwards EM, Horbar JD, Howell EA, McCormick MC, Pursley DM. The color of health: how racism, segregation, and inequality affect the health and well-being of preterm infants and their families. Pediatr Res 2020; 87(2):227-234..

Maternal employment did not have a statistically significant association with premature birth, although a Brazilian cross-sectional study showed lower rates of premature birth among pregnant women who were not employed⁴⁴44 Buen M, Amaral E, Souza RT, Passini R Jr, Lajos GJ, Tedesco RP, Nomura ML, Dias TZ, Rehder PM, Sousa MH, Cecatti JG; Brazilian Multicentre Study on Preterm Birth Study Group. Maternal Work and Spontaneous Preterm Birth: A Multicenter Observational Study in Brazil. Sci Rep 2020; 10(1):9684.. However, women employed, mainly as domestic workers, may have working hours that contribute to inadequate prenatal care regarding the number of appointments, which can increase the frequency of premature birth⁴⁵45 Melo WA, Scardoelli MGC, Iamaguchi K, Carvalho MDB. Influência do Perfil Sócio demográfico Materno nos Prematuros Nascidos no Município de Maringá - PR. In: Anais do VII Encontro Internacional de Produção Científica Cesumar. Maringá: CESUMAR; 2011..

Income and race are correlated with aspects such as where people live, and the interactions among race, education, income, and neighborhood can lead to health care access disparities. Also, psychosocial stress in pregnancy, such as violence and discrimination, can result from where people live, leading to premature birth⁴²42 Burris HH, Hacker MR. Birth outcome racial disparities: A result of intersecting social and environmental factors. Semin Perinatol 2017; 41(6):360-366..

The odds of a baby born prematurely were 2.5 times higher among women with a partner than those without a partner. This finding differs from a Brazilian case-control study, which found that the absence of a stable partner increased by 7.92% the chances of premature birth⁴⁶46 Vieira AS, Mendes PC. Análise Espacial da Prematuridade, Baixo Peso ao Nascer e Óbitos Infantis em Uberlândia - MG. Hygeia 2012; 8(15):146-156.. In addition, the lack of a partner increases the difficulties and responsibilities⁴⁷47 Silva AMR, Almeida MF, Matsuo T, Soares DA. Fatores de risco para nascimentos pré-termo em Londrina, Paraná, Brasil. Cad Saude Publica 2009; 25(10):2125-2138.. Despite the findings of the present study, it is believed that single mothers have an increased risk of adverse birth outcomes, including the occurrence of prematurity⁴⁸48 Sealy-Jefferson S, Butler B, Chettri S, Elmi H, Stevens A, Bosah C, Dailey R, Misra DP. Neighborhood Evictions, Marital/Cohabiting Status, and Preterm Birth among African American Women. Ethn Dis 2021; 31(2):197-204., as marriage can increase access to health services, financial security and social support.

Smoking, alcohol use, and other drugs are amply studied in the literature. They are associated with pregnancy complications since substances ingested during pregnancy cross the placental barrier. Therefore, the fetus is exposed to these substances in the blood, increasing the risk of premature birth⁴⁹49 Fischer A, Melo ECP, Guimarães EC. A Influência de Fatores Sócio-Demográficos na Prematuridade. Rev Pesq Cuidado Fundamental 2010; 2:73-78.^,5050 Moutinho A, Alexandra D. Parto pré-termo, tabagismo e outros fatores de risco - Um estudo caso-controlo. Rev Port Med Geral Familiar 2013; 29(2):107-112.. However, in this study, none of these factors presented a statistically significant relationship with premature birth. These findings diverge from a study conducted in the United States, which observed that low-intensity smoking during pregnancy was associated with an increased risk of premature birth⁵¹51 Silva I, Quevedo LA, Silva RA, Oliveira SS, Pinheiro RT. Associação entre abuso de álcool durante a gestação e o peso ao nascer. Rev Saude Publica 2011; 45(5):864-869..

Regarding the previous history of preterm birth, this study found no relationship with preterm birth in the study period. As long as the occurrence of births at term reduces the risk of premature births in subsequent pregnancies⁵²52 Liu B, Xu G, Sun Y, Qiu X, Ryckman KK, Yu Y, Snetselaar LG, Bao W. Maternal cigarette smoking before and during pregnancy and the risk of preterm birth: A dose-response analysis of 25 million mother-infant pairs. Stock SJ, editor. PLoS Med 2020; 17(8):e1003158., this possibly influenced the low number of women with a history of preterm births in the study sample.

Concerning the type of delivery, most women had a Cesarean section. Even though in 1985 the WHO showed that a rate of Cesarean of more than 15% is medically unjustifiable, the high rates of Cesareans are almost universal⁵³53 Bittar RE, Zugaib M. Indicadores de risco para o parto prematuro. Rev Bras Ginecol Obstetr 2009; 31(4):203-209.. Moreover, type of delivery is a risk factor for premature birth⁵⁴54 Freitas PF, Drachler ML, Leite JCC, Grassi PR. Desigualdade social nas taxas de cesariana em primíparas no Rio Grande do Sul. Rev Saude Publica 2005; 39(5):761-767. which corroborates the findings of this research where those who had a Cesarean section had 3.11 times the chance of premature birth among black women.

The present study has limitations related to the possible biases inherent to epidemiologic investigations, that is, information bias since secondary data were used which can cause underestimation of the prevalence of the outcome; prevalence bias, since data were only collected in the national health services. To minimize these problems, procedures were adopted, such as using a standardized and tested questionnaire, a well-trained team, and standardization in data collection and validation of data by comparing to information obtained. Furthermore, since this research only involved pregnant women in the urban area assisted by the SUS, this may limit the generalization of our findings.

The use of race/ethnicity categories used by IBGE, which the subject self-selects, thus minimizes the bias between the exposed and non-exposed in the study. It is important to emphasize the importance of developing a study on a theme of such relevance but so little studied in the scientific community in Brazil.

The prevalence of premature birth found in this study could be reduced by creating health education programs aimed at prevention and promoting women’s health. Actions such as adequate prenatal visits during pregnancy, health education to clarify the questions of the pregnant women, and controlling risk factors that are already known, among other health promotion initiatives, can reduce the rate of premature births and improve the quality of life of women and newborns. These initiatives should be created universally and equitably to avoid the exclusion of segments of the population and reduce identified racial differences.

In conclusion, this study shows a statistically significant association between maternal race/ethnicity and premature birth. In this context, the existence of racial and social inequalities in the occurrence of premature birth is evident, including the overlap of black women in poverty and lack of access to education³⁵35 Articulação para o Combate ao Racismo Institucional (CRI). Identificação e abordagem do racismo institucional. Brasília: CRI; 2006.. Therefore, studies addressing the issue of race/ethnicity are of great importance in eliminating inequalities in health.

Furthermore, given the evidence found, it is important to state that knowledge about the risk factors associated with the occurrence of prematurity among live births is essential for healthcare management to train the clinicians for preventive actions regarding premature births, as well as to subsidize the planning of measures to promote the health of women of childbearing age. The findings of this study also point to the importance of adequately equipped health services, including the implementation of neonatal ICUs to ensure better survival and quality of life for newborns.

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