Numerous studies in the literature evaluate the association between growth and the infant diet. However, when child age and type of food are specified, the number of studies decreases. Little research has been conducted in children who are exclusively breastfed up to 6 months of life.
Despite the adoption of strategies to encourage breastfeeding in this age group that have resulted in an increased prevalence of exclusive breastfeeding in recent decades, the prevalence rates are still far from ideal [13]. In Belém (PA, Brazil), where this study was conducted, a Brazilian representative survey found that the prevalence of exclusive breastfeeding in infants under six months of age was 56.1 %, which a higher prevalence compared with all state capitals and the Federal District [14].
Knowing the profile of those mothers who were successful with exclusive breastfeeding has been important for identifying future strategies to ensure that children have access to this natural food in the first two years of life. In our study population of exclusively breastfed infants, we observed a high number (90.8 %) of stay-at-home mothers, which corroborates the findings of another study that identified a longer duration of breastfeeding among mothers who stay at home during the first 6 months of the infant’s life [15]. However, low maternal education and maternal unemployment were also associated with an increased risk of early weaning [16].
Queiroz et al. [17], in a cohort study, reported that the absence of maternal cohabitation with her partner and longer EBF are variables that contributed positively and significantly to estimating the average length/age index. If we consider the EBF as a strategy to ensure proper physical growth, these results diverge somewhat from the present study’s findings because more than 70 % of the mothers who cohabited with the children's fathers continued to breastfeed their children, and their children showed adequate growth rates. Indeed, studies have shown that women who are married or living with a partner are more likely to breastfeed than women living alone [18].
Until the emergence of the new WHO, 2006 growth curves [8], the normality standards for children in the first 6 months of life had limited comparability among populations, as not all of the selected sample was EBF and newborns with low birth weights were not excluded [2, 15]. Even for the WHO, 2006 curves, the inclusion criteria required exclusive breastfeeding only until 4 months of age. Therefore, the weight and length distributions found in the present study for EBF children during the first 6 months of life may be considered to be of higher quality because they reflect growth based on the WHO and the American Academy of Pediatrics recommendations that infants in this age group be EBF.
In the United States, the Centers for Disease Control and Prevention (CDC) published a growth curve for American children in 2000 but then went on to recommend the use of the WHO, 2006 growth charts to monitor the growth of children from zero to two years of age [19]. For this reason, we did not use the CDC curves in the present study, although other studies have compared those curves with other references and populations [20].
The comparisons between curves showed that for some age groups, the weight means of the studied sample were higher than the WHO, 2006 curves, despite similar mean values at birth. The means were significantly higher among the male children in their third and fourth months and among the female children in their second, third and sixth months. Such differences most likely arise from the many advantages of exclusive breastfeeding (nutritional, immunological, psychological, emotional, economic and practicality) compared with any other form of feeding during the first 6 months of life [21].
The prevalence of exclusive breastfeeding during the first 6 months of life has increased as a result of increased support and guidance among individuals and groups. The mothers in our study were invited to participate in programs that promote breastfeeding by advocating on-demand breastfeeding and providing adequate technique training and guidance for helping the child properly latch on and suck to obtain the hind milk, which is rich in high-energy fat and thus encourages weight gain. In addition, these programs aim to inform mothers of the advantages that breastfeeding can offer mothers, their children and their families [21, 22].
Although the children in the present study had smaller lengths at birth compared with the curves that were used for comparison, they achieved similar or even higher values in later months, particularly in the second and sixth months for females. These differences can be explained by the sample size (the studied sample was smaller than the one used by WHO, 2006) and by other factors that can interfere and that were not controlled in the present study; the socioeconomic conditions of the constituent sample population, maternal nutrition, nutritional status during pregnancy and pregnancy conditions.
Children of both genders showed a mean that was very close to that expected at six months, which confirms that length during early life, although important, is not especially sensitive to changes in the nutritional status of the children in the studied age group [23]. This peculiarity may also explain the lack of large differences between the growth curves generated here and the WHO, 2006 curves.
Our findings were similar to those of Murahovschi et al. [24], who found curves slightly higher than those defined by the NCHS, 1997 [3]. This similarity can be explained by the fact that both studies considered only EBF children, unlike the NCHS, 1977 curves, in which artificial feeding was predominant. Furthermore, both studies focused on the assessment of children during the first months of life, unlike the NCHS, 1977 curves, which studied a broader age group and had methodological limitations related to the inclusion of children in this initial period of life.
Another study conducted in India partially agrees with the findings of the present study, stating weights were higher in infants who were breastfed compared with the NCHS, 1977 curves; however, the authors observed that length was systematically smaller [25]. In comparison, Jaldin et al. [26], in a Brazilian study of EBF infants up to 6 months of age, concluded that the children’s weight gain resembled the standard WHO, 2006 curves more closely than they resembled the NCHS, 1997 curves, whereas the linear growth was comparable to both curves.
These findings confirm what Augusto and Souza concluded: that exclusive breastfeeding ensures proper growth in the first two quarters of life and that the apparent changes in growth should be evaluated cautiously to avoid early weaning with the introduction of unnecessary and inappropriate complementary feeding [4].
The importance of the inclusion of EBF children in the analysis was that it made it possible to generate a standardised weight and length distribution for the studied age group. Researchers have highlighted the importance of studying EBF children to build appropriate reference growth curves [27].
Furthermore, although the infants evaluated in the present study came from a single geographical area, the sample size was predetermined according to the proposed goal, which increased the possibility of identifying differences between the comparison curves. However, this study was not intended to be a population study or to be representative of the Brazilian population. Thus, the extrapolation of its results to other populations should be conducted with caution [28].
The results of the present study were similar or superior to the reference curves [3, 8], which did not require EBF throughout the first six months of life as inclusion criteria. Additionally, our study’s findings are similar to those of other researchers who studied EBF children in this age group [23, 24, 29].
