Parental body mass index and offspring childhood body size and eating behaviour: causal inference via parental comparisons and extended children of twins structural equation modelling

Background The intergenerational transmission of obesity-related traits could propagate an accelerating cycle of obesity, if parental adiposity causally influences offspring adiposity via intrauterine or periconceptional mechanisms. We aimed to establish whether associations between parental peri-pregnancy body mass index (BMI) and offspring birth weight (BW), BMI until 8 years and 8-year eating behaviour are due to genetic confounding. Methods We used data from the Norwegian Mother, Father and Child Cohort Study and the Medical Birth Registry of Norway. We compared the strength of the associations of maternal versus paternal BMI with offspring outcomes, and used an extended children of twins structural equation model (SEM) to quantify the extent to which associations were due to genetic confounding (n = 17001 to 85866 children). Findings Maternal BMI was more strongly associated than paternal BMI with offspring BW, but the maternal-paternal difference decreased for offspring BMI after birth. Greater parental BMI was associated with obesity-related offspring eating behaviours. SEM results indicated that genetic confounding did not explain the association between parental BMI and offspring BW, but explained the majority of the association with offspring BMI from 6 months onwards. For 8-year BMI, genetic confounding explained 79% (95% CI: 62%, 95%) of the covariance with maternal BMI and 94% (95% CI: 72%, 113%) of the covariance with paternal BMI. Interpretation We found strong evidence that parent-child BMI associations are primarily due to genetic confounding, arguing against a strong causal effect of maternal or paternal adiposity on childhood adiposity via intrauterine or periconceptional mechanisms.