Previous meta-analyses suggested that cesarean section is associated with increased BMI in offspring
Confounding is a concern, even when adjusting for BMI.One approach is to use within-family sibling data to control for environmental factors and shared genetic background.Rifas-Shiman et al. (JAMA Pediatrics, 2018) sought to determine whether there is an association between mode of delivery and BMI of offspring

           So a Retrospective cohort study was conducted where Data derived from the Linked CENTURY (Collecting Electronic Nutrition Trajectory Data Using Records of Youth) study.The Model adjusted for Maternal age  | Parity| Race & ethnicity,Child’s age | Sex | Birth year.

              Data analyzed to assess within-family component and between-family component and Main exposure: was route of delivery.Primary outcomes was  Age-specific and sex-specific BMI z-score (also called standard deviation score) at 5 years of age.BMI z-scores are measures of relative weight adjusted for child age and sex
0.0 would represent 50th percentile BMI while 2.5 would be >99th percentile
       Result was asfollows:-----
16,140 siblings were included from 8070 mothers
19.9% of children had cesarean delivery
Mean (SD) BMI z-score at 5 years of age was 0.48 (1.00)
Mean BMI z-score was
0.45 among siblings who both had vaginal delivery
0.51 among siblings with 1 cesarean and 1 vaginal delivery
0.63 among siblings who both had cesarean delivery
In co-variate adjusted model, the within-family association of cesarean vs vaginal delivery was not significant
0.04 higher BMI z-score at 5 years of age (95% CI, −0.04 to 0.11)
The addition of the between-family assessment changed association to 0.13 (95% CI, 0.04 to 0.22)
In a model adjusted for the same covariates, but without decomposing the within-family and between-family effects, children with cesarean delivery had 0.13 higher BMI z score (95% CI, 0.08 to 0.17)
CONCLUSION:
The authors found that within families, cesarean section was not associated with increase in BMI z-score at 5 years of age.The authors conclude that previous studies, which found association with obesity and increased weight was limited by confounding