A high prevalence of depressive symptoms was detected in the present study among children from the less developed city, São Luís. In multivariable analysis in São Luís, lower maternal schooling and young paternal age were independently associated with the presence of depressive symptoms. In Ribeirão Preto, the more developed city, low birth weight and socioeconomic variables (skilled and semi-skilled manual occupation and unskilled occupation and unemployment of the family head) were risk factors for presence of depressive symptoms.
Prevalence of depressive symptoms
According to the American Academy of Child and Adolescent Psychiatry, the prevalence of depression among children and adolescents aged 9 to 17 years has been estimated at 5% . Using the 27-item CDI, a 11.6% prevalence of possible depression in children aged 11 to 15 years was reported in Northern Ireland using a cut-off point of 17 . In Brazil, using the 20-item CDI and 17 as the cut-off point, prevalences of 1.48% were detected among children aged 7 to 14 years in a private school in Ribeirão Preto , of 22% among schoolchildren aged 7 to 17 years in a city in Paraíba, Brazil . Using the 27-item CDI and the same cut-off point, the prevalence of possible depression was 13.7% among schoolchildren aged 7 to 13 years in a school in Minas Gerais, Brazil , and of 28.5% among 10- to 17-year-old pupils of a public school in Curitiba, state of Paraná, Brazil . In the present study, using the 27-item CDI and 17 as the cut-off point, the prevalence of depressive symptoms among São Luís children (21.9%) was approximately equal to that detected in Paraíba, Brazil, using the 20-item CDI with the same cut-off point . This prevalence was lower than that detected in another Brazilian study conducted in Minas Gerais that used the 27-item CDI with the same cut-off point . In Ribeirão Preto, the prevalence was much lower, with a value close to those reported in international studies [33, 39] and higher than that reported in a study with a similar cut-off point and age range in the same city . In international studies, however, the methods and cut-off points vary considerably [34, 40], hampering comparisons.
Several investigations have demonstrated a wide variation in the prevalence of depression in children and differences according to the criteria of diagnostic classification adopted, mainly due to the diverse mode of presentation of depression and the association of this disorder with other psychopathologies. In addition, there still is no single instrument for the evaluation of depression . Among the inventories of self-evaluation of depressive symptoms, the CDI is the one most extensively used and was adapted to Brazil by Gouveia et al. in 1995 .
Other factors that impair comparison of the results are the different age ranges of the populations studied and the time and location of the studies. More recent studies  have reported higher prevalences of depressive symptoms compared to older studies . In addition, studies including older children  have reported higher prevalences than studies on younger children . The choice of the sample to be studied can also influence the results since small samples are imprecise and convenience samples, limited to certain public or private schools, are not representative of the target populations [35, 37].
The prevalence of depressive symptoms detected in São Luís is similar to that observed by Barbosa and Gaião  in Paraíba, a fact that can be explained by the similar socioeconomic and demographic conditions of the two locations. Both São Luís-MA and João Pessoa-PB are located in a less developed region of Brazil, with the highest socioeconomic unequalities in almost all health indicators among children younger than five years. This creates a situation of inequity that makes it difficult for the children to reach their full potential regarding health and physical and mental development , possibly explaining the higher prevalences detected in Brazilian studies conducted in less developed regions .
Low birth weight and depressive symptoms
In Ribeirão Preto, the prevalence of depressive symptoms was higher among low birth weight (<2500 g) children compared to normal birth weight children. This result agrees with other studies that showed that LBW, including VLBW (500–1499 g), may influence the development of depression in later life [6, 17, 18, 24]. In multivariable analysis, in Ribeirão Preto, the association between low birth weight and the presence of depressive symptoms remained significant even after adjusting for preterm birth, indicating that intrauterine growth restriction rather than preterm birth might explain this association. Similar results were found in a study in Helsinki , where intrauterine growth restriction (IUGR) was also identified as a risk factor for depression.
In São Luís, there was no association between LBW and the presence of depressive symptoms, in agreement with other international studies [20–23]. Mortality in the first year of life was higher in São Luís than in Ribeirão Preto, a fact that may have provoked survival bias and may explain why low birth weight was not associated with depressive symptoms in São Luís. In São Luís, few children with VLBW, which could have been more prone to experiencing depressive symptoms, survived . In addition, oversampling of LBW children was more successful in Ribeirão Preto than in São Luís.
It is known that when children approach adolescence their depressive symptoms tend to resemble those of adults . This may have contributed to explaining why the association between LBW and symptoms of depression was only found in the most developed city, Ribeirão Preto, where children were older.
In Ribeirão Preto, in a model including birth weight, treated as a continuous variable, and a quadratic term for birth weight, association between birth weight and depressive symptoms was non-linear, being higher in the lower birthweights, decreasing from 500 g to 3500 g and increasing thereafter, suggesting that risks of depressive symptoms are higher for both low and high birth weight groups.
Few studies have been published on the association between LBW and depression at school age [7, 20, 24]. Most studies involved older populations such as young adults , middle-aged adults , or elderly persons [5, 9, 17, 22, 23]. The variety of methods used in previous studies for the evaluation of depression may have been a factor contributing to the conflicting results detected in the literature and in the present study.
Social factors and depressive symptoms
A worse socioeconomic situation assessed on the basis of maternal schooling and occupation of family head was an important predictor of the presence of depressive symptoms in both cities, in agreement with other studies [5, 11, 18]. Manual occupation or unemployment of the family head has also been associated with depressive symptoms in some studies [11, 18, 45]. These findings suggest that disadvantaged social environments may have adverse consequences on mental health because of their effects on psychological development . In multivariable analysis, lower maternal schooling was a risk factor for the presence of depressive symptoms in São Luís. The lower the educational level of the parents, the worse tend to be the physical and emotional conditions for the development and stimulation of the children [10, 11, 17], since these parents are assumed to have less access to information and therefore interact poorly with their children.
Young paternal age and depressive symptoms
In our study young paternal age (<20 years) was independently associated with increased depressive symptoms in the less developed city, São Luis . Some young fathers present low level of emotional maturity and lower financial stability, with consequent lower emotional support for their child. This may predispose their children to depression . However, the association between young paternal age and mental health problems has been little researched. In one study low paternal age has been associated with poor mental health, as measured by the Strenghts and Difficulties Questionnaire . Nevertheless, most studies identified advanced but not young paternal age as a risk factor for psychiatric disorders, including schizophrenia [46, 49]. In one recent study, young paternal age was associated with increased odds of major depressive disorder, in agreement with our finding in the less developed city . In contrast, in the more developed city we did not find an association between young paternal age and depressive symptoms. It is possible that psychosocial strains associated with young paternal age were buffered in the more developed city. This association suggests that increasing paternal age at pregnancy might reduce depressive symptoms.
Sex and depressive symptoms
Sex was not associated with the presence of depressive symptoms, in contrast to some international studies, which reported that depression was more prevalent among adolescent or adult females [5, 9]. This finding may be explained by the fact that children in our sample were aged 7–11, in contrast to most studies, which included adolescents or adults. It seems that in young children depression does not differ according to sex, whereas as they enter adolescence girls tend to report higher rates of depression than boys .
Strengths and limitations
This was one of the few cohort studies carried out to assess the prevalence of risk factors for depressive symptoms in children in a middle-income country, by comparing a more developed city to a less developed one. The over-representation of the groups of high and low birth weight was a strategy used to increase the statistical power of the study.
The high rates of loss due to mortality, migration and failure to locate many children was a limitation of the study. Participation rates were lower in Ribeirão Preto for mothers who were less than 20 years of age (p = 0.005), who cohabited (p < 0.001) or had ≤ 4 years of full time education (p < 0.001) in comparison to eligible children who did not participate. In São Luís, a lower proportion of children were born to mothers with ≥ 12 years of full time education (p < 0.001), who were primiparous (p = 0.049) or gave birth to males (p = 0.001) among those who participated in the study in comparison to the eligible group who did not participate . These differences may have led to an overestimate of the prevalence of depressive symptoms in São Luís, where the follow-up rate for the group with high schooling was very low.
The CDI is a screening test. Although it can serve as an aid in the diagnostic process, it cannot yield a diagnosis by itself . It could be that the high prevalence of depressive symptoms found in our study, especially in the less developed city, São Luís, was due to the fact that we used a screening instrument. However, the cut-off point 20 used in the present study is recommended when the CDI is used as a general population-based screen test, to minimize false positives .
The age difference of two years may have contributed to explaining part of the differences in depressive symptoms between the two cohorts. Children from São Luís were a little younger than those from Ribeirão Preto and may have had difficulties in describing and evaluating their feelings in the last fortnight, as asked by the CDI. A study has shown that the reliability of the child's report of the Diagnostic Interview Schedule for Children (DISC) increased with age and was lower for children aged 6–9 than for those aged 10–13 and 14–18 . However, the prevalence of depression tends to increase with age and thus higher prevalences would be expected in Ribeirão Preto, where children were older. Another limitation was that we only used self-reporting of depressive simptoms and did not assess teacher or parental reports.