Mode of delivery and adiposity: Hong Kong's “Children of 1997” birth cohort
Introduction
The increasing prevalence of childhood obesity is of public health concern, given its related adverse health consequences [1], [2]. Childhood obesity tends to track into adulthood, where it is a well-established risk factor for diabetes and cardiovascular disease [3], [4]. Given the difficulty of achieving sustained weight loss, preventive strategies are urgently needed and may be informed by the etiology of childhood obesity [4]. Over the last 30 years, cesarean section rates have increased, coinciding with the increase in obesity prevalence [5]. Some recent observational studies found cesarean section associated with higher body mass index (BMI) or higher risk of overweight in children from 3 to 15 years [6], [7], [8]. Infants born vaginally acquire their gut flora from maternal vaginal and fecal flora, whereas for the infants born by cesarean sections, the environment is an important source of gut flora [9]. Due to lack of contact with the maternal or vaginal microbiota, infants born by cesarean sections have a different composition of gut microbiota from vaginally born infants, with more microbes from the Firmicutes family and fewer from the Bacteroidetes family [10], [11]. This composition may contribute to the development of early childhood obesity [12] because gut microbiota differ with weight status and changing the gut microbiota can affect weight status [13]. Obesity is associated with fewer intestinal Bacteroidetes and more Firmicutes, which are better at harvesting energy from the diet [14]. Weight loss increases Bacteroidetes and decreases Firmicutes [15].
The association of cesarean section with overweight or obesity has not been consistently observed [16], [17], [18]. Discrepancies may be driven by biases arising from different social patterning of cesarean section and obesity across settings [19], [20], [21] and methodological limitations such as high attrition rates. Given the equivocal evidence to date, evidence from other sociohistorical contexts with different social patterns of behavior can be valuable.
In 1985, the World Health Organization (WHO) recommended an upper limit to the cesarean section rate of 15%. High cesarean section rates are associated with higher mortality and morbidity in mothers and babies [22]. Hong Kong has a high cesarean section rate [5], which was 27.4% in 1999 [23]. Elective cesarean sections mostly occur in private hospitals (45% of births), as public hospitals do not provide cesarean sections without clinical indication (∼17% of births) [23]. In a large, contemporary, population-representative Hong Kong Chinese birth cohort, “Children of 1997,” where cesarean section is associated with higher socioeconomic position (SEP) [24] but the social patterning of childhood obesity is unclear [25], we examined the association of mode of delivery (vaginal or cesarean) with BMI and the presence of overweight (including obesity) into late childhood. We also examined if the associations varied by birth hospital. Cesarean sections in public hospitals are unplanned and might occur during labor when exposure to vaginal microbiota has occurred; elective cesarean sections in private hospitals usually occur before the onset of labor avoiding exposure to vaginal microbiota.
Section snippets
Source of data
The Hong Kong “Children of 1997” birth cohort (N = 8327) covered 88.0% of all births from April 1, 1997 to May 31, 1997. The study was initially established to investigate the effect of secondhand smoke (SHS) exposure on infant health [26], [27]. Families were recruited at the first postnatal visit to any of the 49 Maternal and Child Health Centers in Hong Kong [26], which parents of all newborns are encouraged to attend for free postnatal care, developmental checks, and vaccinations until the
Results
About one-quarter of births (26%) were cesarean sections, 17% were assisted vaginal births and 57% were unassisted vaginal births. The proportion of cesarean sections was 48% in private hospitals and 17% in public hospitals. The percentage overweight (including obesity) from 3 to 13 years ranged from 6.9% to 23.9% (Fig. 2). Cesarean section was associated with male sex, lower gestational age, lower parity, less paternal smoking, less breastfeeding, higher maternal BMI, higher maternal age,
Discussion
In this large, population-representative birth cohort of Chinese children from a developed non-Western setting, mode of delivery was not clearly associated with BMI z-score or overweight (including obesity) into late childhood. We also found no strong evidence that the associations varied by the type of birth hospital, where the indications for a cesarean section might differ or by growth phase.
Our finding is consistent with three cohort studies from Brazil, Denmark, and Canada [16], [17], [18]
Conclusions
In this population-representative birth cohort from a non-Western developed setting with little socioeconomic patterning of childhood obesity, mode of delivery was not clearly associated with BMI or overweight (including obesity) into late childhood, although we cannot rule out the possibility of a role for gut microbiota. Further studies are needed to confirm our findings, especially the role of unplanned cesarean sections, setting specific composition of gut microbiota, and dietary influences
Acknowledgment
The authors thank colleagues at the Student Health Service and Family Health Service of the Department of Health for their assistance and collaboration. They also thank Dr. Connie Hui for her assistance with the record linkage and the late Dr. Connie O for coordinating the project and all the fieldwork for the initial study in 1997–1998.
Funding: This work is a substudy of the “Children of 1997” birth cohort which was initially supported by the Health Care and Promotion Fund, Health and Welfare
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Cited by (0)
Contributor's statement: Study concept and design: S.L.L and C.M.S.; Acquisition of data: C.M.S. and G.M.L.; Analysis and interpretation of data: S.L.L and C.M.S.; Drafting of the manuscript: S.L.L and C.M.S.; Obtained funding: C.M.S. and G.M.L.; Study supervision: C.M.S. and G.M.L.