Estimation of Preterm Birth Rate, Associated Factors and Maternal Morbidity From a Demographic and Health Survey in Brazil
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To determine the prevalence of preterm birth from self-reports by Brazilian women, to assess complications, interventions and outcomes, to identify factors associated with preterm birth, and to improve the preterm birth rates estimates. This is a secondary analysis of data from a Demographic Health Survey. It interviewed a sample of 4,743 Brazilian women who had 6,113 live births from 2001 to 2007. Estimates of preterm birth rates were obtained per region and per year according to self-reported gestational age. The prevalence rate and 95 % confidence interval (CI) for preterm was determined according to the characteristics of mothers and offspring. Odds ratios and 95 % CI were estimated for complications such as severe maternal morbidity. The preterm birth rate was 9.9 %, with regional variations. Preterm birth was more likely to be associated with neonatal death, low birth weight, and longer hospital stay. Maternal factors associated with preterm birth were: white ethnicity, living in an urban area, history of hypertension or heart disease, twin gestation, non-elective Cesarean section, medical insurance for delivery, low number of antenatal visits, and severe morbidity. A self-report survey has indicated that the preterm birth rate in Brazil is higher than official data suggest, with an increasing trend in more developed areas, and is associated with poor neonatal and maternal outcomes.
KeywordsPreterm birth Risk factors Prematurity Demographic and health survey Severe maternal morbidity
Body mass index
Demographic health survey
Live births information system
World Health Organization
Neonatal care has markedly improved over a number of years, resulting in decreased infant mortality rates, especially in developing countries . Many countries, however, fall a long way short of the ‘Millennium Development Goals’, proposed by the United Nations . An increase in preterm births in the last two decades has raised alarm among health policy-makers, including those in developed countries.
Neonates born before fetal maturity subsequently require more health care. In addition, many diseases in adulthood are directly associated with the intrauterine living conditions (Barker Theory) . Thus, delivery occurring before the fetus has reached maturity denotes a potential fetal risk, which may possibly result in short or long term consequences.
The most recent and largest study on the topic reported that the incidence of preterm birth rose in the last two decades in the great majority of countries and also that Brazil contributed as the 10th largest number of preterm newborns worldwide . Another study in the USA showed that, in 2006, 12.8 % of births were preterm, an increase of 21 % compared to 1990 . In 2008, at the Surgeon General’s Conference on the Prevention of Preterm Birth in the United States, many experts concluded that preterm represents both social impairment and a serious public health problem, and is becoming more prevalent worldwide . In fact, a recent systematic review estimated 12.9 million preterm births globally for 2005 alone, which would account for 9.6 % of all births worldwide .
Preterm, traditionally defined as birth occurring before 37 weeks of gestation , is still the leading cause of neonatal morbidity and mortality throughout the world. It is also the major cause of deficiencies acquired after birth, in particular those involving neurological abnormalities . With a multifactorial origin, preterm birth can occur between 22 and 37 weeks, and becomes markedly more severe as the gestational age at birth decreases. Many of the risk factors associated with preterm birth result in inflammation, thus increasing uterus stimulation, such as in local and systemic infections. Some risk factors are associated with maternal sociodemographic characteristics, including African-American ethnicity, low socioeconomic and educational status, young and advanced maternal ages, single marital status, working long hours, and a difficult labor under stress. Other risk factors include maternal psychological stress, tobacco use, alcohol abuse and drug use, pregnancy history (close temporal proximity to a previous delivery and previous preterm birth), actual pregnancy conditions (multiple gestation, vaginal bleeding, polyhydramnios or oligohydramnios), and existing medical conditions, such as low body mass index (BMI), thyroid disease, asthma and diabetes .
Currently, the maternal medical condition is one of the most important risk factors, and therapeutic preterm birth is an important proportion of the total number of preterm births. Some obstetric conditions, such as hemorrhage and hypertension, are reported as risk factors for preterm birth; for instance, gestational age at birth is usually lower when associated with preeclampsia [11, 12]. Some authors have suggested that maternal morbidity accounts for almost half of preterm births . Another risk factor identified by some authors is related to the mode of delivery. Although Cesarean section is intrinsically associated with complications, there is also an increased risk of unintended iatrogenic prematurity [13, 14] and of neonatal mortality and morbidity .
In order to reduce preterm births, it is first important to determine the prevalence, and potential risk factors associated with it. In Brazil, official data show a mean prevalence of roughly 6.5 % , below the rates reported by developed countries, and relatively close to the estimate for South America of 7.9 % . This surprisingly low rate may reflect the difficulty of adequately determining gestational age at birth, as well as the likely under-reporting of births, including those preterm, especially in the less developed regions of the country. Indeed, differentiating the proportion of low birth weight newborns that are preterm is extremely difficult where there is no standardized estimation of gestational age. A recent systematic review on the subject found that in low and middle income countries, roughly half of all low birth weight newborns were preterm . Therefore a problem still exists regarding the estimates of preterm birth in settings where the estimation of real gestational age is not standardized nor is the information easily available for great majority of women delivering.
Since 1986, a national population survey has been conducted every 10 years in Brazil to determine the demographic and health profile of the female population of childbearing age and of children under 5 years of age. Such a study can allow an assessment of how maternal and child health has developed in recent years in Brazil. Therefore, the purpose of this study was to assess the self-reported prevalence of preterm birth in a sample of Brazilian women interviewed in the Demographic and Health Survey (DHS) performed in 2006 . The study also aimed at assessing the conditions associated with the health of women who had preterm deliveries, the quality of care received during pregnancy, the occurrence of complications and interventions among pregnant women and their outcomes. Characteristics of preterm and term pregnancies were compared to identify possible factors associated with preterm, and also to compare the data with the official data provided on preterm births in Brazil.
Materials and Methods
This study is a secondary analysis of the DHS data acquired in Brazil in 2006–2007. These data are currently in the public domain . The original study was conducted according to the ethical standards established by the Declaration of Helsinki and approved by the National Review Board. The period of data collection was from 31st October 2006 to 3rd May 2007. The survey was representative of the national population, with assessments in five regions of the country in both rural and urban areas. A total of 15,575 women, aged 15–49 years, were interviewed. In the current analysis, we considered women who had at least one live birth since 2001. The total number of live births during this period in the five regions of the country was 6,160. Thus, data of 4,743 women were extracted from the database. For 47 live births, however, there was no information in the database on the occurrence of preterm birth. All information on twin live births was considered.
The original data of this study was obtained electronically, directly from the website of the Ministry of Health of Brazil, as the database is in the public domain. Additionally, data on the official total number of live births, including those for gestational age groups, for Brazil and its regions were also obtained via the internet. Thus, official preterm birth estimates were compared with the results from the DHS.
For data analysis, initially a descriptive bivariate analysis by region of residence of women was performed. We used the number of pregnancies occurring from 2001 and the live births according to the gestational age as reported by the mother. The prevalence of preterm births was evaluated by region of residence for the period 2001–2007, with information from two sources, the DHS and the live births information system (SINASC) from the Ministry of Health. We stratified the DHS data according to preterm, defined as less than 37 weeks gestation, or term delivery. Associations were then tested using the Pearson Chi-square test. Various factors were assessed, including sociodemographic characteristics, maternal clinical conditions, and features related to pregnancy, prenatal care, and delivery. The analysis determined crude estimates or prevalence rate (PR) with 95 % confidence interval (CI) and also estimates adjusted by other predictors with Poisson multiple regression (PRadj). Finally, the live births were assessed according to maternal complications and/or interventions, or both, with crude odds ratio (OR) and 95 % CI reported as an indirect measure of any association. The characteristics of the complex sampling method (stratum, clustering, weighting) were performed in all analyses. For the analysis, SPSS (version 17.0) and Stata (version 7.0.) softwares were used.
Percentage distribution of women with at least one live birth child from 2001 to 2007, according to the number of pregnancies by region (weighted analysis)
Number of pregnancies from 2001 to 2007
Percentage distribution of live birth children from 2001 to 2007, according to self-reported gestational age at birth by region (weighted analysis)
Self-reported gestational age (months)
Prevalence of preterm with weighting (%)†
Prevalence of preterm live births according to the year of birth by region from 2001 to 2007, using two information sources
Prevalence of preterm with weighting (DHS)
(Total live births)a
Official data (SINASC‡)
Percentage distribution of women with a live birth child from 2001 to 2007, according to characteristics, grouped by preterm and term births (weighted analysis)
PR (95 % CI)
PRadj. (95 % CI)c
Until primary school
> primary school
Total household income
Until R$ 500.00
Hypertension or heart disease
Percentage of live birth children from 2001 to 2007, according to medical factors, grouped by preterm and term births (weighted analysis)
PR (95 % CI)
RPadj (95 % CI)c
Not scheduled or not Cesarean
Place of prenatal careb
Number of prenatal care visits
Up to 5
Characteristics of live birth children from 2001 to 2007, grouped by preterm and term delivery (weighted analysis)
Child dead at the survey date
Low birth weight
Children remained hospitalized after maternal discharge
Reason for hospitalization
Hospital stay (≥ 8 d) of children who were hospitalized after maternal discharge
Percentage of live birth children from 2001 to 2007, according to complications and/or interventions in mothers, grouped by preterm and term delivery (weighted analysis)
Complications and interventions in mothers
OR (95 % CI)
Eclampsia (seizures during pregnancy, birth or postpartum without previous occurrences)
Hemorrhage (heavy bleeding during the first 3 days postpartum)
Infection (high fever after delivery, with chills and smelly vaginal discharge)
Admission to ICU
Length of hospital stay >1 week
Using information from a secondary analysis of a population household survey, this study is perhaps the first attempt to use data from a DHS to estimate national data on preterm births and their regional differences. It also identified a mean prevalence of preterm births in Brazil, for the last 6 years preceding the survey among all women interviewed, of around 10 % of live births, with significant regional variations. The highest variation was in the south and the lowest was in the north-east. Over the study period, however, there were no significant changes within any region.
The analysis also allowed a correlation between the prevalence of preterm birth with maternal morbidity. The study found that women with preterm delivery were more often white, living in urban areas and had a history of hypertension. In addition, preterm births were associated with increased risk of neonatal death, low birth weight, hospitalization, longer hospital stay, twin pregnancies, Cesarean section, and fewer prenatal care visits.
Data obtained from this study indicated a prevalence of preterm birth of 9.9 % in Brazil. Although these data represent only self-reported information of the women interviewed (hence such information is not from medical records), these results, however, substantially exceeded the 6.5 % officially reported for the country . Paradoxically, these data are very close to those reported by developed countries, in which the birth registries are conducted more systematically and therefore more reliably. This suggests that the real prevalence of preterm births is not exactly known in Brazil.
This enormous disparity between the SINASC data and hospital records had already been observed by other authors. By using a linkage method, Silva et al.  observed low concordance (Kappa = 0.09) among the preterm records when using the SINASC and a hospital survey. Generally, Brazilian studies have shown poor reliability in estimates of gestational age, by comparing the SINASC with other sources of data, with Kappa ranging from 0.09 to 0.83 .
Other studies also showed similar results, with the SINASC prevalence always lower than the prevalence obtained by other methods [19, 21, 22, 23]. However, more recent studies found only a minor difference between the SINASC and the method used, which may be a result of an improvement in the information reliability of SINASC . The difference in this study between SINASC data and DHS data could be the result of the way the gestational age is represented in the declaration of live births for the SINASC. This document does not report the exact gestational age but relatively wide intervals of age (less than 22 weeks, 22–27, 28–31, 32–36, 37–41, 42 or more). This could be a source of error if a gestational age close to term, for instance a 36 weeks baby, was wrongly classified as at term, then considering as preterm delivery only the cases of extreme gestational ages. This would lead to an under-reporting of cases of late preterm birth from 34 to 36 weeks, which would be classified as term .
As the study collected information from the past last 5–6 years, a potential recall bias in interviewing the women cannot be overlooked. This bias, however, would not explain the lower prevalence in the official registry. Recall bias could theoretically reduce the prevalence of the condition for the years more distant from the interview, and this may be so, as there was a trend toward a rise in the estimated prevalence of preterm births over recent years, as was observed by other authors [4, 5, 20]. In this study, such an increase was observed when comparing the first triennium (2001–2003) to the second triennium (2004–2006/7).
There were some marked regional variations, with higher prevalence in southern and southeastern regions, in both the SINASC data and the DHS data. It can be observed, however, that there was an increase in the overall preterm birth rate in both the DHS and in the SINASC data, the latter showing an increase from 5 % in 1994 to 5.4 % in 1998, 5.6 % in 2000, 6.6 % in 2005, and 6.7 % in 2007. Such an increase in the prevalence may represent an improvement in the reliability of data collection [20, 25]. Another relevant explanation could be the improvements in obstetric care with more at risk infants being delivered live preterm. The following conditions related to the possibility of live preterm delivery may be recognized: (a) improved access to health services and consequent reduction in delay to identify and treat risk factors; and (b) improvement in conditions for obstetric and perinatal care, consequently with a higher rate of therapeutic preterm after maternal complications .
Brazil has been experiencing an increase in the coverage of prenatal care and an improvement in obstetric and perinatal care, with easier access to such services . Therefore, a rise in the prevalence of preterm births could possibly be a consequence of better treatment for maternal complications and more babies surviving in utero. Some of the findings in this study concerning the characteristics and risk factors for preterm births support this hypothesis. White women living in urban areas, who are users of health insurance, are part of a population with greater access to health services. In addition, women with private insurance are known to be at higher risk for preterm birth for several reasons, including wealth, higher access to infertility treatments and also the consequent multiple pregnancies. The lower number of prenatal visits, associated with preterm, could be a consequence of shorter pregnancies.
Other factors associated with preterm births (twins, hypertension, Cesarean section) could be related to an increase in morbid conditions of pregnant women, except for the variable ‘Cesarean scheduled’, which appears as a protective factor for preterm birth, probably because there is no medical indication to schedule an elective Cesarean section before term. Thus, maternal morbidity is the major factor which explains the increase in prevalence of preterm births. In fact, eclampsia increased the risk of preterm birth by 4.5 times. Other indicators of severe maternal morbidity, e.g., hemorrhage, admission to intensive care units, inter-hospital transfer, need for mechanical ventilation, and prolonged hospitalization also increased the risk. The presence of any indicator of severe maternal morbidity or of a life-threatening condition during gestation increased the risk of preterm nearly 2.5 times in the study.
Other authors have already reported an association between clinical and gestation conditions and preterm birth [4, 10, 26]. However, this seems to be the first study to identify an association of life-threatening conditions and severe maternal morbidity (maternal near miss), as recently defined by WHO , with preterm births in a sample representative of the whole female population in a large country.
As far as we know, this study is also the first to use a DHS to analyze data related to preterm, and this may bring some limitations to the study. The accuracy of self-reported data for the diagnosis of preterm birth is not yet known. Likewise, as the information depends on the interviewees’ memory, a recall bias may occur, although it is unlikely for such a major life event. In addition, the sampling process selected nearly 6,000 women for a population representative of 17 million people. As the primary objective of the study was not to identify the prevalence of preterm births, the power of this analysis may have been reduced, generating some distortions.
This study, however, presented results which suggest a trend for a rise in the prevalence of preterm births in Brazil. Furthermore, it identified factors associated with preterm birth, and, in particular, showed for the first time that preterm delivery was more frequent among women who have any type of severe maternal morbidity. Strategies used to promote improvements in healthcare for a population should prioritize high-risk situations which occur more frequently and/or with greater severity, or both. In order to do so, knowledge of the prevalence of adverse clinical conditions which affects the target population, is vital. For this, further detailed studies on the topic are warranted .
Finally, although there are concerns regarding the validity of self-reporting of preterm births, it may be a useful method for epidemiological analysis, especially in developing countries, where official detailed collection of such data is still limited. If the validity can be confirmed, the possibility of assessing the prevalence of preterm births in the last two or three decades in all countries that implemented a DHS would be presented. In addition, if these analyses could also identify possible risk factors associated with preterm birth, then this could help in regional policy-making to reduce the recent observed increase in the global prevalence of preterm delivery.
This manuscript was performed with the support from the grant 2009/53245-5 from Fapesp.
Conflict of interest
The authors declare they have no conflict of interests.
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