In this population-based cross-sectional study, we found that the overall prevalence of childhood asthma, AR, urticarial, FA and DA among children aged 6–11 years was 13.9%, 22.7%, 15.3%, 8.1% and 4.6%, respectively, in Shanghai, China. Male sex, high SES, CS delivery, only one child in the household and having family history of allergy were among the major risk factors for childhood asthma and allergic diseases. The longer exclusive breastfeeding duration (༞6 months) was a protective factor for childhood asthma and allergic diseases, and could also attenuate the adverse effects of CS delivery, only one child in the household and having family history of allergy on childhood asthma and allergic diseases.
Our findings are consistent with most previous studies in which CS delivery, only one child in the household and formula feeding were associated with an increased risk of allergic diseases among children [6–8, 12–18]. Recently, a systematic review and meta-analysis reported that male sex, short duration of breastfeeding and having siblings were risk factors of early transient wheezing among children aged 3–18 years) . Our findings also show that male sex and short duration of breastfeeding were risk factors for childhood asthma and allergic diseases (Table 1), but in contrast, only one child (having no siblings) in the household was a risk factor for childhood asthma and allergic diseases. In addition, Chu et al  reported that breastfeeding attenuated the impacts of CS on childhood asthma and AR. However, Liao et al  suggest that the association of CS with developing childhood allergy was not modified by breastfeeding duration. Our results provide supportive evidence that breastfeeding duration modifies the association between CS and childhood allergy. The inconsistency between studies may be due to the differences in geographic locations, study designs and population characteristics.
Hygiene or old friends hypotheses involved gut microbiota exposure might partly explain the associations of CS, only one child in the household, and breastfeeding with childhood asthma and allergic diseases . A meta-analysis reported an increase in the risk of childhood asthma after CS (OR = 1.20, 95% CI: 1.14, 12.6) . Birth by CS causes development of the gut microbiota to be delayed and to take an unusual course . Only one child, without siblings, also modulates the gut microbiota, leading to allergic disorders [23, 24]. However, breastfeeding could prevent allergy through regulating infant gut barrier function and microbiota , which may explain why longer breastfeeding duration attenuated the adverse effects of CS and only one child in the household on childhood asthma and allergic diseases. CS might boost inflammatory responses, affect bronchial epithelial barrier function, or associate with the metabolic syndrome, or both. In turn, breastfeeding could strengthen bronchial epithelial barrier function and boost innate immunity and regulatory immune responses .
Similar to previous studies [2, 17, 19, 27–29], we found that family history of allergy was strongly associated with the risk of childhood asthma and allergic diseases. Longer breastfeeding duration could reduce these risks. The World Health Organization issued a recommendation that mothers should breastfeed their children exclusively for 6 months at least . Exclusive breastfeeding is the internationally preferred method of feeding babies during their first 6 months of life, and is recognized as one of the most natural and best forms of preventive medicine.
There are three major strengths in this study. First, a representative sample was obtained through a stringent multi-stage and multi-strata random sampling approach. Second, this population-based cross-sectional study achieved a high response rate (༞95%), so selection bias was probably minimal. Third, a wide range of childhood allergic diseases including asthma, AR, urticarial, FA and DA were considered in this study to explore their associations with CS, only one child in the household, family history of allergy and breastfeeding duration.
This study also has several limitations. First, recall bias was inevitable to some extent as all the information was obtained from questionnaires. We repeated the survey twice in a school to compare the accuracy and differences of the recalled information, and observed the high quality of collected data (Note: we provided some details in the Methods section). Second, the nature and severity of childhood asthma and allergic diseases was not measured in this study, and therefore, the determinants of mild and severe cases cannot be distinguished. Finally, a causal relationship cannot be established due to the cross-sectional study design. However, the independent variables included in the multivariable model (e.g., child’s sex, CS, only one child in the household and family history of allergy) were significantly earlier than the diagnosis of childhood asthma and allergic diseases, so a certain temporal relationship exists.
Despite these limitations, the results from this study have demonstrated the contemporary prevalence of asthma and allergic diseases among children aged 6–11 years and their associations with CS, only one child in the household, family history of allergy and breastfeeding duration. Longer breastfeeding duration appeared to decrease the adverse effects of the neonatal and familiar risk factors on childhood asthma and allergic diseases. Our findings may be used to develop appropriate strategies to prevent and control childhood asthma and allergic diseases if they are confirmed by further research.