Abstract

INTRODUCTION

In many countries, the prevalence of preterm births, occurring before the 37^thweek of pregnancy, ^aaMarch of Dimes; The Partnership for Maternal Newborn and Child Health; Save the Children; World Health Organization. Born too soon: the global action report on preterm birth. Geneva: WHO; 2012. shows a tendency to increase, even in high-income countries such as the United States, Australia, Japan and Canada. ¹³13 . Lawn JE, Gravett MG, Nunes TM, Rubens CE, Stanton C. Global report on preterm birth and stillbirth (1 of 7): definitions, description of the burden and opportunities to improve data. BMC Pregnancy Childbirth . 2010;10(Suppl 1):S1. DOI:10.1186/1471-2393-10-S1-S1
https://doi.org/10.1186/1471-2393-10-S1-... A World Health Organization Report, ²⁸28 . World Health Organization. Born too Soon. The Global Action Report on Preterm Birth. Geneva: WHO; 2012. based on estimates made through statistical modelling, estimates that in the world, every year, there are 15 million preterm births, more than 10.0% of the total. ^aaMarch of Dimes; The Partnership for Maternal Newborn and Child Health; Save the Children; World Health Organization. Born too soon: the global action report on preterm birth. Geneva: WHO; 2012.

This high prevalence represents complications related to prematurity, the most common cause of neonatal and infant death in middle- and high-income countries ¹³13 . Lawn JE, Gravett MG, Nunes TM, Rubens CE, Stanton C. Global report on preterm birth and stillbirth (1 of 7): definitions, description of the burden and opportunities to improve data. BMC Pregnancy Childbirth . 2010;10(Suppl 1):S1. DOI:10.1186/1471-2393-10-S1-S1
https://doi.org/10.1186/1471-2393-10-S1-... including Brazil. ⁴4 . Barros FC, Bhutta ZA, Batra M, Hansen TN, Victora CG, Rubens CE. Global report on preterm birth and stillbirth (3 of 7): evidence for effectiveness of interventions. BMC Pregnancy Childbirth . 2010;10 Suppl 1:S3. ^{, 8}8 . Goldani MZ, Barbieri MA, Rona RJ, Silva AAM, Bettiol H. Increasing pre-term and low-birth-weight rates over time and their impact on infant mortality in south-east Brazil. J Biosoc Sci . 2004;36(2):177-88. DOI:10.1017/S0021932003006175
https://doi.org/10.1017/S002193200300617... ^{, 27}27 . Victora CG, Aquino EM, Leal MC, Monteiro CA, Barros FC, Szwarcwald CL. Maternal and child health in Brazil: progress and challenges. Lancet . 2011;377(9780):1863-76. DOI:10.1016/S0140-6736(11)60138-4
https://doi.org/10.1016/S0140-6736(11)60... The high prevalence of prematurity has significant social and economic repercussions: in the short term, increasing demand for neonatal intensive care units and the costs of the care needed, in the long term, for individuals suffering from permanent health conditions with irreparable damage to their physical and mental health. ¹³13 . Lawn JE, Gravett MG, Nunes TM, Rubens CE, Stanton C. Global report on preterm birth and stillbirth (1 of 7): definitions, description of the burden and opportunities to improve data. BMC Pregnancy Childbirth . 2010;10(Suppl 1):S1. DOI:10.1186/1471-2393-10-S1-S1
https://doi.org/10.1186/1471-2393-10-S1-...

The WHO report places Brazil as the country with the tenth highest number of preterm births in absolute terms, ^aaMarch of Dimes; The Partnership for Maternal Newborn and Child Health; Save the Children; World Health Organization. Born too soon: the global action report on preterm birth. Geneva: WHO; 2012. with an estimated prevalence of 9.2%. Official data collected by the Sistema de Informações sobre Nascidos Vivos (SINASC – Live Birth Information System) between 2000 and 2010 indicate that there has been a slight increase in the prevalence of preterm births in the country (from 6.8% to 7.1%).

Data collected in Brazilian studies ²⁶26 . Silveira MF, Santos IS, Barros AJD, Matijasevich A, Barros FC, Victora CG. Increase in preterm births in Brazil: review of population-based studies. Rev Saude Publica . 2008;42(5):957-64. DOI:10.1590/S0034-89102008000500023
https://doi.org/10.1590/S0034-8910200800... using primary data suggest that the prevalence of preterm births is higher than the level estimated by the SINASC and that the increase in prematurity appears to be higher in recent decades. A systematic review (2008) ²⁶26 . Silveira MF, Santos IS, Barros AJD, Matijasevich A, Barros FC, Victora CG. Increase in preterm births in Brazil: review of population-based studies. Rev Saude Publica . 2008;42(5):957-64. DOI:10.1590/S0034-89102008000500023
https://doi.org/10.1590/S0034-8910200800... showed an evolution in the prevalence of prematurity from around 4.0% at the beginning of the 1980s to over 10.0% after 2000. Reliable, validated SINASC studies indicate low concordance with the rate of preterm births obtained based on rates calculated in population based research. ¹⁹19 . Pedraza DF. Qualidade do Sistema de Informações sobre Nascidos Vivos (Sinasc): análise crítica da literatura. Cienc Saude Coletiva . 2012;17(10):2729-37. DOI:10.1590/S1413-81232012001000021
https://doi.org/10.1590/S1413-8123201200... This poor concordance may be partly explained by the fact that until 2011, data on gestational age (GA) was collected in class intervals and not in complete weeks. ¹⁹19 . Pedraza DF. Qualidade do Sistema de Informações sobre Nascidos Vivos (Sinasc): análise crítica da literatura. Cienc Saude Coletiva . 2012;17(10):2729-37. DOI:10.1590/S1413-81232012001000021
https://doi.org/10.1590/S1413-8123201200...

The aim of this study was to estimate the prevalence of preterm births according to birth weight and to obtain an equation to correct the estimates.

METHODS

A systematic review of Brazilian population based publications from 2008, containing information on the prevalence of preterm births from 1970 to 2004. ²⁶26 . Silveira MF, Santos IS, Barros AJD, Matijasevich A, Barros FC, Victora CG. Increase in preterm births in Brazil: review of population-based studies. Rev Saude Publica . 2008;42(5):957-64. DOI:10.1590/S0034-89102008000500023
https://doi.org/10.1590/S0034-8910200800... The search included articles published in journals, dissertations and theses. This review was updated in 2012 for the 1990-2012 period, using the Medline and Lilacs databases, using the search terms: (premature/preterm and Brazil); (premature/preterm delivery and Brazil); (premature/preterm infant and Brazil); (premature/preterm labor and Brazil); (risk factors and premature/preterm delivery and Brazil); (risk factors and premature/preterm labor and Brazil); (associated factors and premature/preterm labor and Brazil); (associated factors and premature/preterm delivery and Brazil); (incidence and premature/preterm labor and Brazil); (prevalence and premature/preterm labor and Brazil); (incidence and premature/preterm delivery and Brazil); (prevalence and premature/preterm delivery and Brazil).

Articles concerning clinical topics, such as complications of prematurity and pregnancy, or health care for premature newborns were excluded. Inclusion criteria were: studies on the prevalence of prematurity in Brazil carried out after 1990. Studies deemed to have representative samples were those which included all births taking place in a specific location, in a specific period or which used some kind of probabilistic process to select a sample of newborns. In the case of more than one article based on the same database, the first to be published was included.

Forty-nine references were identified in the Medline database in the 2008 review, ²⁶26 . Silveira MF, Santos IS, Barros AJD, Matijasevich A, Barros FC, Victora CG. Increase in preterm births in Brazil: review of population-based studies. Rev Saude Publica . 2008;42(5):957-64. DOI:10.1590/S0034-89102008000500023
https://doi.org/10.1590/S0034-8910200800... of which 42 were excluded and ten included. In the Lilacs database, 46 references were identified, with three being included (two articles and one thesis) and 43 excluded as they did not meet the inclusion criteria. The references of all the articles were examined, although no further articles were identified. Finally, 11 texts were identified using primary data on the Northeast, South and Southeast of the country: nine articles published in journals, ¹1 . Almeida SDM, Barros MBA. Equidade e atenção à saúde da gestante em Campinas (SP), Brasil. Rev Panam Salud Publica . 2005;17(1):15-25. DOI:10.1590/S1020-49892005000100003
https://doi.org/10.1590/S1020-4989200500... ^{, 5}5 . Barros FC, Huttly SR, Victora CG, Kirkwood BR, Vaughan JP. Comparison of the causes and consequences of prematurity and intrauterine growth retardation: a longitudinal study in southern Brazil. Pediatrics . 1992;90(2 Pt 1):238-44. ^{, 7}7 . Bettiol H, Rona RJ, Chinn S, Goldani M, Barbieri MA. Factors associated with preterm births in southeast Brazil: a comparison of two birth cohorts born 15 years apart. Paediatr Perinat Epidemiol . 2000;14(1):30-8. DOI:10.1046/j.1365-3016.2000.00222.x
https://doi.org/10.1046/j.1365-3016.2000... ^{, 9}9 . Gray RH, Ferraz EM, Amorim MS, Melo LF. Levels and determinants of early neonatal mortality in Natal, northeastern Brazil: results of a surveillance and case-control study. Int J Epidemiol . 1991;20(2):467-73. ^{, 11}11 . Horta BL, Barros FC, Halpern R, Victora CG. Baixo peso ao nascer em duas coortes de base populacional no Sul do Brasil. Cad Saude Publica . 1996;12(Supl 1):S27-31. DOI:10.1590/S0102-311X1996000500005
https://doi.org/10.1590/S0102-311X199600... ^{, 15}15 . Lunardelli AN, Peres MA. Is there an association between periodontal disease, prematurity and low birth weight? A population-based study. J Clin Periodontol . 2005;32(9):938-46. DOI:10.1111/j.1600-051X.2005.00759.x
https://doi.org/10.1111/j.1600-051X.2005... ^{, 17}17 . Nascimento LFC. Epidemiology of preterm deliveries in Southeast Brazil: a hospital-based study. Rev Bras Saude Mater Infant . 2001;1(3):263-8. DOI:10.1590/S1519-38292001000300007
https://doi.org/10.1590/S1519-3829200100... ^{, 21}21 . Rondo PH, Ferreira RF, Nogueira F, Ribeiro MC, Lobert H, Artes R. Maternal psychological stress and distress as predictors of low birth weight, prematurity and intrauterine growth retardation. Eur J Clin Nutr . 2003;57(2):266-72. DOI:10.1038/sj.ejcn.1601526
https://doi.org/10.1038/sj.ejcn.1601526... ^{, 23}23 . Silva AAM, Coimbra LC, Silva RA, Alves MTSSB, Lamy Filho F, Lamy ZC, et al. Perinatal health and mother-child health care in the municipality of São Luís, Maranhão State, Brazil. Cad Saude Publica . 2001;17(6):1413-23. DOI:10.1590/S0102-311X2001000600025
https://doi.org/10.1590/S0102-311X200100... one doctoral thesis ^bbRumel D. Acurácia dos critérios de risco do Programa de Defesa da Vida dos Lactentes do Município de Bauru entre 1986 e 1988 [Tese]: Universidade de São Paulo; 1989. and one Master’s dissertation. ^ccOliveira MT. A saúde da mulher trabalhadora: estudo da relaçäo entre trabalho na gestaçäo e a ocorrência de doenças, complicaçäo do parto e recém-nascidos prematuros na cidade do Recife, Pernambuco [Dissertação]: Universidade Federal da Bahia; 1992.

The bibliographical research was widened to include publications from after 2004 and to exclude those published before 1990. The bibliographical research used the same methodology described in the previous study. ²⁶26 . Silveira MF, Santos IS, Barros AJD, Matijasevich A, Barros FC, Victora CG. Increase in preterm births in Brazil: review of population-based studies. Rev Saude Publica . 2008;42(5):957-64. DOI:10.1590/S0034-89102008000500023
https://doi.org/10.1590/S0034-8910200800... This procedure identified two new publications^{2, 10}10 . Gurgel RQ, Nery AMDG, Almeida MLD, Oliveira ERR, Lima DDF, Bettiol H, et al. Características das gestações, partos e recém-nascidos da região metropolitana de Aracaju, Sergipe, Brasil. Rev Bras Saude Mater Infant . 2009;9(2):167-77. DOI:10.1590/S1519-38292009000200006
https://doi.org/10.1590/S1519-3829200900... and four articles were excluded. ⁵5 . Barros FC, Huttly SR, Victora CG, Kirkwood BR, Vaughan JP. Comparison of the causes and consequences of prematurity and intrauterine growth retardation: a longitudinal study in southern Brazil. Pediatrics . 1992;90(2 Pt 1):238-44. ^{, 7}7 . Bettiol H, Rona RJ, Chinn S, Goldani M, Barbieri MA. Factors associated with preterm births in southeast Brazil: a comparison of two birth cohorts born 15 years apart. Paediatr Perinat Epidemiol . 2000;14(1):30-8. DOI:10.1046/j.1365-3016.2000.00222.x
https://doi.org/10.1046/j.1365-3016.2000... ^{, 9}9 . Gray RH, Ferraz EM, Amorim MS, Melo LF. Levels and determinants of early neonatal mortality in Natal, northeastern Brazil: results of a surveillance and case-control study. Int J Epidemiol . 1991;20(2):467-73. ^{, 22}22 . Royston P, Sauerbrei W. Multivariable Model-building: A Pragmatic Approach to Regression Analysis Based on Fractional Polynomials for Modelling Continuous Variables. Chichester, UK: Wiley 2008.

The authors of these nine publications were contacted and invited to participate. They were asked to provide the following information on the database used in the publication: the total number of newborns and preterm newborns for each birth weight group, divided per 100 g, starting at 400 g. This data was to be reported separately, according to sex.

For four studies identified in the review, ¹1 . Almeida SDM, Barros MBA. Equidade e atenção à saúde da gestante em Campinas (SP), Brasil. Rev Panam Salud Publica . 2005;17(1):15-25. DOI:10.1590/S1020-49892005000100003
https://doi.org/10.1590/S1020-4989200500... ^{, 2}2 . Araújo BF, Bozzetti MC, Tanaka ACA. Mortalidade neonatal precoce no municipio de Caxias do Sul: um estudo de coorte. J Pediatr (Rio J). 2000;76(3):200-6. ^{, 17}17 . Nascimento LFC. Epidemiology of preterm deliveries in Southeast Brazil: a hospital-based study. Rev Bras Saude Mater Infant . 2001;1(3):263-8. DOI:10.1590/S1519-38292001000300007
https://doi.org/10.1590/S1519-3829200100... ^{, c}cOliveira MT. A saúde da mulher trabalhadora: estudo da relaçäo entre trabalho na gestaçäo e a ocorrência de doenças, complicaçäo do parto e recém-nascidos prematuros na cidade do Recife, Pernambuco [Dissertação]: Universidade Federal da Bahia; 1992. the authors reported that the original database no longer existed, and the article could therefore not be included in the analyses, leaving seven studies.

The authors of the articles and other Brazilian researchers in this field were asked whether they were aware of any other Brazilian databases, published or not, the authors of which could provide the data presented. At the end of this process, another five databases were identified. Four of them concerned original, unpublished studies: BRISA – cohorts in Ribeirao Preto, SP, and Sao Luís, MA, both from 2010; Rio de Janeiro, RJ, 2010; Pelotas, RS, Intergrowth 2010, ²⁵25 . Silveira MF, Barros FC, Sclowitz IK, Domingues MR, Mota DM, Fonseca SS, et al. Implementation of the INTERGROWTH-21st Project in Brazil. BJOG : an international journal of obstetrics and gynaecology. 2013;120 Suppl 2:81-6. and the fifth was a collaborative nationwide study. ¹⁴14 . Leal MC, Silva AAN, Dias MAB, Gama SGN, Rattner D, Moreira ME, Theme Filha MM, et al. Birth in Brazil: national survey into labour and birth. Reprod Health . 2012;9:15. DOI:10.1186/1742-4755-9-15
https://doi.org/10.1186/1742-4755-9-15... At the end of the search, twelve databases were available for the research.

A linear regression model based on fractional polynomials was constructed to estimate the expected proportion of premature births for each birth weight group. Modelling using fractional polynomials is ideal in this situation in which the study was not linear, as it allows a high degree of flexibility in the adjusted curves. This approach is superior to the traditional way of adding quadratic, cubic and other terms to the model. ²²22 . Royston P, Sauerbrei W. Multivariable Model-building: A Pragmatic Approach to Regression Analysis Based on Fractional Polynomials for Modelling Continuous Variables. Chichester, UK: Wiley 2008. This strategy of selecting curves uses a set of pre0defined exponents (-2, -1, -0.5, 0, 0.5, 1, 2, 3), where zero equals the natural logarithm. The independent variable may appear more than once in the regression equation; therefore, a model with two terms is selected. Of the 44 possible models (eight with only one term, and another 36 combinations of the eight exponents in pairs), the best was chosen. The proportion of premature births varied between 0 and 1 and the outcome underwent a logistic transformation before the model was adjusted. In order to avoid infinites in the result of the transformation in cases where the proportion was zero or one, 0.01 was added or subtracted. The mean value of the birth weight for each interval was used as a predictor. After adjusting the models, separate estimates for males and females, the adjusted values and their respective confidence intervals were transformed again to the original scale. The models were adjusted to give proportional weighting to the number in each birth weight group for each study. Each of these studies was treated as one conglomerate in calculating the standard error of the coefficients. ²²22 . Royston P, Sauerbrei W. Multivariable Model-building: A Pragmatic Approach to Regression Analysis Based on Fractional Polynomials for Modelling Continuous Variables. Chichester, UK: Wiley 2008.

Data referring to 2000, 2005 and 2010 from the SINASC 2000-2011 database, when the data on gestational age were collected in categories (0-21, 22-27, 28-36, 37-41, 42 weeks and over, or unknown) were analyzed. From 2011 onwards, this data started to be collected as a continuous variable, measured in weeks. The same analyses carried out in the studies of primary data were repeated with the SINASC data.

RESULTS

Table shows the data concerning the databases, authors, year, location of the study, number of children taking part, method for determining gestational age and prevalence of preterm births and of newborns with low birth weight.

Six of the databases concern repeated studies conducted in Sao Luís, MA (1997-1998 and 2010, the latter as yet unpublished), ²³23 . Silva AAM, Coimbra LC, Silva RA, Alves MTSSB, Lamy Filho F, Lamy ZC, et al. Perinatal health and mother-child health care in the municipality of São Luís, Maranhão State, Brazil. Cad Saude Publica . 2001;17(6):1413-23. DOI:10.1590/S0102-311X2001000600025
https://doi.org/10.1590/S0102-311X200100... Ribeirao Preto, SP (1994 and 2010, the latter also unpublished) ⁷7 . Bettiol H, Rona RJ, Chinn S, Goldani M, Barbieri MA. Factors associated with preterm births in southeast Brazil: a comparison of two birth cohorts born 15 years apart. Paediatr Perinat Epidemiol . 2000;14(1):30-8. DOI:10.1046/j.1365-3016.2000.00222.x
https://doi.org/10.1046/j.1365-3016.2000... and Pelotas, RS (1993, 2004, 2011). ³3 . Barros AJD, Santos IS, Victora CG, Albernaz EP, Domingues MR, Timm IK, et al. Coorte de nascimentos de Pelotas, 2004: metodologia e descrição. Rev Saude Publica . 2006;40(3):402-13. DOI:10.1590/S0034-89102006000300007
https://doi.org/10.1590/S0034-8910200600... ^{, 11}11 . Horta BL, Barros FC, Halpern R, Victora CG. Baixo peso ao nascer em duas coortes de base populacional no Sul do Brasil. Cad Saude Publica . 1996;12(Supl 1):S27-31. DOI:10.1590/S0102-311X1996000500005
https://doi.org/10.1590/S0102-311X199600... These studies are population based, including birth occurring in these municipalities (generally over the period of one year) or systematic samples of all newborns. In the Ribeirao Preto and Sao Luís cohorts in the 2010-BRISA, births in households resident in the municipalities were included. Pelotas, RS, contributed with a perinatal study carried out over a 15-month period between 2011 and 2012, evaluating all births in the municipality. This study was part of a multi-center international project – Intergrowth-21. ²⁵25 . Silveira MF, Barros FC, Sclowitz IK, Domingues MR, Mota DM, Fonseca SS, et al. Implementation of the INTERGROWTH-21st Project in Brazil. BJOG : an international journal of obstetrics and gynaecology. 2013;120 Suppl 2:81-6. The perinatal study in Aracaju, SE, in 2005, ¹⁰10 . Gurgel RQ, Nery AMDG, Almeida MLD, Oliveira ERR, Lima DDF, Bettiol H, et al. Características das gestações, partos e recém-nascidos da região metropolitana de Aracaju, Sergipe, Brasil. Rev Bras Saude Mater Infant . 2009;9(2):167-77. DOI:10.1590/S1519-38292009000200006
https://doi.org/10.1590/S1519-3829200900... assessed births in the four largest maternity wards in the municipality over four months. Births occurring in 2010 in the maternity school of the Universidade Federal do Rio de Janeiro were studied by Cunha (data not published). Two other studies included in the sample had some type of selection of the population studied: one in Itajaí, SC, ¹⁵15 . Lunardelli AN, Peres MA. Is there an association between periodontal disease, prematurity and low birth weight? A population-based study. J Clin Periodontol . 2005;32(9):938-46. DOI:10.1111/j.1600-051X.2005.00759.x
https://doi.org/10.1111/j.1600-051X.2005... on periodontal disease and preterm births, with data collected daily for five months in the city’s only maternity ward, the sample including data on pregnant women aged 18 to 40, without significant health problems, and with at least 18 natural teeth. The other cohort study, in Rondó, ²¹21 . Rondo PH, Ferreira RF, Nogueira F, Ribeiro MC, Lobert H, Artes R. Maternal psychological stress and distress as predictors of low birth weight, prematurity and intrauterine growth retardation. Eur J Clin Nutr . 2003;57(2):266-72. DOI:10.1038/sj.ejcn.1601526
https://doi.org/10.1038/sj.ejcn.1601526... Jundiaí, SP, on stress during pregnancy and perinatal results, included women receiving full pre-natal care in health care units and hospitals in the municipality.

This review included unpublished data regarding the Nascer no Brasil study, ¹⁴14 . Leal MC, Silva AAN, Dias MAB, Gama SGN, Rattner D, Moreira ME, Theme Filha MM, et al. Birth in Brazil: national survey into labour and birth. Reprod Health . 2012;9:15. DOI:10.1186/1742-4755-9-15
https://doi.org/10.1186/1742-4755-9-15... which is a multi-centric hospital based cohort that included a post-natal visit and a telephone interview between 45 and 60 days postpartum. The sample was stratified by geographical macro-region, type of municipality and type of hospital. The data were collecetd between February 2011 and October 2012 in hospitals with at least 500 births per year, and data on hospital births, all live births and still births weighing at least 500g and/or with a gestational age of > 22 weeks were included. In this study, newborns with birth weights incompatible with GA (< -3 SD or > 3 SD in the weight curve for WHO GA) were excluded from the analysis. ¹⁶16 . Mikolajczyk RT, Zhang J, Betran AP, Souza JP, Mori R, Gülmezoglu AM, et al. A global reference for fetal-weight and birthweight percentiles. Lancet. 2011;377(9780):1855-61. DOI:10.1016/S0140-6736(11)60364-4
https://doi.org/10.1016/S0140-6736(11)60...

In all of the studies, the data of the last menstrual period (LMP) was the method used to calculate gestational age, sometimes complemented by ultrasound (US) or physical examination of the newborn. In the studies in Ribeirao Preto and Sao Luís, unknown gestational ages were imputed using a multiple regression model. Even those studies which used several sources of gestational age (such as Intergrowth, ²⁵25 . Silveira MF, Barros FC, Sclowitz IK, Domingues MR, Mota DM, Fonseca SS, et al. Implementation of the INTERGROWTH-21st Project in Brazil. BJOG : an international journal of obstetrics and gynaecology. 2013;120 Suppl 2:81-6. in Pelotas, or Nascer no Brasil ¹⁴14 . Leal MC, Silva AAN, Dias MAB, Gama SGN, Rattner D, Moreira ME, Theme Filha MM, et al. Birth in Brazil: national survey into labour and birth. Reprod Health . 2012;9:15. DOI:10.1186/1742-4755-9-15
https://doi.org/10.1186/1742-4755-9-15... ), the date of the last menstruation was used to increase comparability with the SINASC.

Lower prevalence was observed in the studies in Jundiaí and Itajaí. The prevalence of preterm births in the studies conducted after 2000 was higher than 10.0%, except the study in Aracaju, in 2005, in which the prevalence was 7.7% and the study in Itajaí, in 2003, with a prevalence of 7.1%. The lowest rates of prevalence of low birth weight among recent studies were observed in municipalities in the Northeast – Aracaju (7.2%) and Sao Luís (8.6%) –, whereas studies in the South and Southeast had rates of prevalence between 9.5% (Ribeirao Preto, 2010) and 12.4% (Pelotas, 2011). The national study Nascer no Brasil ¹⁴14 . Leal MC, Silva AAN, Dias MAB, Gama SGN, Rattner D, Moreira ME, Theme Filha MM, et al. Birth in Brazil: national survey into labour and birth. Reprod Health . 2012;9:15. DOI:10.1186/1742-4755-9-15
https://doi.org/10.1186/1742-4755-9-15... had an estimate of 8.5% ( Table ).

The number of births in each 100 g category was small in some of the studies. Variability was observed, especially up to a weight of 2,000 g, which included relatively few children ( Figure 1 ).

Fractional polynomials were calculated based on these data ( Figure 2 ). For males and females, the model selected was to the power of 2 and 3, using the logit outcome of the proportion of premature births in each birth weight group. The mean value of birth weight in each interval was used as a predictor, giving the following equation for males:

And for females the equation was:

The resulting curves remained practically unchanged when the analysis was repeated without weighting, i.e., with the same weight on each of the studies ( Annexes 1 and 2 ).

The primary studies indicated higher rates of prevalence of preterm births than the SINASC in practically all birth weight groups between 1,000 g and 3,200 g ( Figures 3 and 4 ). The change in the mode of SINASC data collection in 2011, when gestational age began to be collected as a continuous variables, resulted in greater concordance with the results of the primary studies in birth weight groups of 2,000g and upwards.

Applying correction factors to the SINASC database for 2010 indicated a national prevalence of 11.7%, higher than the SINASC figure of 7.1% ( Annex 3 ).^d

DISCUSSION

The studies included represent practically all of the existing research on the epidemiology of premature birth in the period in question. With the exception of the nationwide Nascer no Brasil study, ¹⁴14 . Leal MC, Silva AAN, Dias MAB, Gama SGN, Rattner D, Moreira ME, Theme Filha MM, et al. Birth in Brazil: national survey into labour and birth. Reprod Health . 2012;9:15. DOI:10.1186/1742-4755-9-15
https://doi.org/10.1186/1742-4755-9-15... the studies were carried out in the Southeast, South and Northeast.

Different methods can be used for assessing gestational age in epidemiological studies. High quality US examinations in the first 14 weeks represent the gold standard. ¹⁸18 . National Institute for Health Care and Excellence. Antenatal care: routine care for the healthy pregnant women. London; 2008. (NICE Clinical Guidelines, 62). However, the majority of epidemiological studies are based on LMP data, even in countries such as the United States and the United Kingdom, where the difference between the two methods is between two and three days. ¹²12 . Kramer MS, Papageorghiou A, Culhane J, Bhutta Z, Goldenberg RL, Gravett M, et al. Challenges in defining and classifying the preterm birth syndrome. Am J Obstet Gynecol . 2012;206(2):108-12. DOI:10.1016/j.ajog.2011.10.864
https://doi.org/10.1016/j.ajog.2011.10.8... Two Brazilian studies present conflicting visions of this topic. A prospective study in two cities in the Southeast of public health care system – Sistema Único de Saúde (SUS) users ²⁰20 . Pereira AP, Dias MA, Bastos MH, Gama SG, Leal MC. Detemining gestational age for public health care users in Brazil: comparison of methods and algorithm creation. BMC Research Notes . 2013;6:60. showed that, compared with results obtained by ultrasound (US) up to 20 weeks, the date of the last menstrual period (after excluding outliers) had a sensitivity of 71.0% and accuracy of 94.0%, estimating the prevalence at 14.0% compared with 12.5% according to US. The authors considered the US as the gold standard, without discussing the quality of the examination and mentioned that in one of the two municipalities, they were performed by a health care professional connected to the research team. The 2004 cohort study in Pelotas, ⁶6 . Barros FC, Victora CG, Barros AJ, Santos IS, Albernaz EP, Matijasevich A, et al. The challenge of reducing neonatal mortality in middle-income countries: findings from three Brazilian birth cohorts in 1982, 1993, and 2004. Lancet . 2005;365(9462):847-54. DOI:10.1016/S0140-6736(05)71042-4
https://doi.org/10.1016/S0140-6736(05)71... uses LMP as the gold standard. It shows that, for infants born between 32 and 36 weeks, US carried out before 20 weeks overestimated gestational age by 1.8 weeks for pregnant women cared for by the SUS, although this figure was only five days in the private sector. This difference is attributed to the poor quality of the exams carried out on SUS patients. LMP was used as the gold standard in this study, in contrast with the previous study.

Although early US is the gold standard when performed properly by a trained professional, there are many Brazilian women for whom this data is not available. The mean age of the first US exam available was 20.0 weeks, with a standard deviation of 7.6 in the study in Pelotas (2004). ⁶6 . Barros FC, Victora CG, Barros AJ, Santos IS, Albernaz EP, Matijasevich A, et al. The challenge of reducing neonatal mortality in middle-income countries: findings from three Brazilian birth cohorts in 1982, 1993, and 2004. Lancet . 2005;365(9462):847-54. DOI:10.1016/S0140-6736(05)71042-4
https://doi.org/10.1016/S0140-6736(05)71... As the main objective of this study was to compare data with that of the SINASC, where gestational age is usually measured using hospital charts and based on LMP, it was decided to use data based on LMP of the various primary studies, although some provided data based on US or physical examination of the newborn.

The underlying assumption of combining the data of the studies in diverse regions, specifically, that the prevalence of intrauterine growth restriction would be relatively constant throughout the country, justified combining data from different regions in one single estimate. Although, historically, the North and Northeast have worse health indicators for mother and baby health, this differential is being rapidly reduced. ²⁷27 . Victora CG, Aquino EM, Leal MC, Monteiro CA, Barros FC, Szwarcwald CL. Maternal and child health in Brazil: progress and challenges. Lancet . 2011;377(9780):1863-76. DOI:10.1016/S0140-6736(11)60138-4
https://doi.org/10.1016/S0140-6736(11)60... The prevalence of low weight/height and height/age ratios in children under five was practically the same for the whole country in 2006-07. ²⁷27 . Victora CG, Aquino EM, Leal MC, Monteiro CA, Barros FC, Szwarcwald CL. Maternal and child health in Brazil: progress and challenges. Lancet . 2011;377(9780):1863-76. DOI:10.1016/S0140-6736(11)60138-4
https://doi.org/10.1016/S0140-6736(11)60... Low birth weight, paradoxically, is slightly more prevalent in richer regions than in poorer regions, ²³23 . Silva AAM, Coimbra LC, Silva RA, Alves MTSSB, Lamy Filho F, Lamy ZC, et al. Perinatal health and mother-child health care in the municipality of São Luís, Maranhão State, Brazil. Cad Saude Publica . 2001;17(6):1413-23. DOI:10.1590/S0102-311X2001000600025
https://doi.org/10.1590/S0102-311X200100... and the same is true of the prevalence of preterm births estimated by SINASC. ^ddMatijasevich A et al, unpublished data. The current analyses show that the confidence intervals of the estimates obtained using fractional polynomials are narrow ( Figures 3 and 4 ), with the exception of birth weight groups < 2,000 g, in which the number of births in the studies is very small. However, the results of these studies appear to be homogenous and there is no evidence of differences between regions that impede national estimates being obtained, which justifies the approach used.

All births with birth weight < 1,000 g were preterm, prevalence > 90.0% in infants between 1,000 g and 1,800 g. From 3,000 g and upwards, the prevalence of preterm births was low. As in many studies, the numbers of births in each 100 g group were small, there is considerable variation between the studies, but the mean curves follow the expected pattern. The relatively low rates of prevalence observed in Jundiaí and Itajaí are probably due to the fact that these studies included samples of pregnant women who probably had lower risk pregnancies.

Comparing the curve based on the primary data and the SINASC results suggests that the SINASC underestimates the prevalence of premature births in Brazil. The situation appears to have changed for the better in 2011 compared with the previous decade. This is possibly due to the fact that gestational age began to be collected in exact weeks, and not grouped into categories. Even in 2011, many maternity wards used the previous version of the Statement of Live Birth, which is being gradually replaced by the updated version. A more exact evaluation of the impact of the change in the document may be able to be carried out from 2012 onwards. The differences between the SINASC data and the primary data occur mainly up to 3,000 g. Above this weight, they are lower and not significant.

These analyses suggest that the SINASC data underestimated the prevalence of preterm births at least until 2010. In this year, the estimate of 7.1% reported by the SINASC was 385 lower than the corrected prevalence of 11.7%. Preliminary results of the Nascer no Brasil study, ¹⁴14 . Leal MC, Silva AAN, Dias MAB, Gama SGN, Rattner D, Moreira ME, Theme Filha MM, et al. Birth in Brazil: national survey into labour and birth. Reprod Health . 2012;9:15. DOI:10.1186/1742-4755-9-15
https://doi.org/10.1186/1742-4755-9-15... based on ultrasounds, indicate that national prevalence of 11.4%, close to the estimate presented here.

We suggest that the non-corrected rates of prevalence based on SINASC data not be used. The curves shown here can be used to correct the SINASC results and obtain more precise estimates of the prevalence of preterm births for different geographic areas and risk groups.

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