Researchers found that while maternal BMI affects birth weight, genetics account for most parent-child BMI similarities throughout childhood, challenging the idea that obesity risk is largely programmed before birth.
Study: Parental body mass index and offspring childhood body size and eating behaviour: A structural equation modelling analysis in the Norwegian Mother, Father and Child Cohort Study. Image credit: NDAB Creativity/Shutterstock.com
A recent study published in the journal PLoS Medicine suggests that maternal obesity affects birth weight more strongly than it does childhood BMI, whose association with parental BMI is better explained by shared genetics.
Large family study explores childhood obesity inheritance
Childhood obesity and overweight remain highly prevalent across high-income countries. This is associated with both biological and social factors, including family eating habits and physical activity. It is also familial in nature, linked to parental body mass index (BMI), and develops early in life, prompting interest in preventive interventions targeting parents.
It is important to understand how obesity-related traits are transmitted, as a direct biological effect of parent BMI on child BMI might accelerate the development of obesity in successive generations. The interaction of genetic versus environmental factors in such transmission remains unclear.
Multiple studies suggest a strong effect of genetic traits on parent-child BMI associations, but this has not been accurately quantified. However, an alternative explanation is the developmental overnutrition hypothesis, which suggests that parental obesity affects offspring development preconceptionally or during intrauterine life. Paternal obesity could affect offspring metabolism through direct or indirect genetic effects on sperm and seminal fluid.
The heritability of adiposity also introduces genetic confounding into the association. Environmental confounding could also occur due to the high impact of shared family socioeconomic context, eating habits, and physical activity behaviors.
The current study aimed to examine whether genetic factors explain the association between higher parental body mass index (BMI) around the time of conception and offspring birth weight, childhood BMI, and eating behavior through age 8. The study drew on the Norwegian Mother, Father and Child Cohort Study (MoBa) linked to the Medical Birth Registry of Norway. It included up to 85,866 parent-child pairs depending on the specific analysis.
The researchers used an extended Children of Twins (the Multiple Children of Twins and Siblings [MCoTS] structural equation model) to analyze associations between related family members. This model separates the exposure-outcome associations into genetic confounding and a residual non-genetic component that may include causal effects and environmental confounding. This allows the researchers to assess whether the parent-child BMI associations were explained by genetic inheritance rather than intrauterine or periconceptional effects of parental BMI on obesity-linked traits. If so, reducing parental BMI alone, without changing the child’s postnatal environment, may be insufficient to substantially reduce childhood obesity risk.
Maternal BMI had a unique influence on birth weight
The researchers first compared how maternal and paternal BMI related to their children's birth weight. They found that maternal BMI showed a much stronger association than paternal BMI, and the structural equation model indicated that this difference could not be explained by shared genetics alone.
Instead, the findings support an association between maternal BMI and fetal growth, suggesting that the biological environment during pregnancy plays a role in determining birth weight.
Childhood BMI was largely explained by genetics
The picture changed after birth. Although children with parents who had higher BMI were themselves more likely to have higher BMI, maternal and paternal BMI showed remarkably similar associations with childhood BMI beyond early infancy. This pattern suggested that shared inherited factors, rather than pregnancy-specific effects, were driving much of the relationship.
The structural equation model reinforced this conclusion. From six months of age onwards, most of the association between parental and offspring BMI was explained by genetic confounding, and this contribution remained consistently high throughout childhood. By eight years of age, shared genetic inheritance accounted for 79% of the association between maternal and offspring BMI and 94% of the association with paternal BMI.
Although the researchers note that some estimates may have been affected by statistical uncertainty, the overall pattern remained consistent: as children grew older, the association between parental and child BMI strengthened, while the proportion explained by shared genetics remained high.
Taken together, these findings suggest that much of the relationship between parental and childhood BMI reflects inherited genetic susceptibility rather than a substantial developmental programming effect of maternal obesity during pregnancy, although a smaller causal effect cannot be ruled out. Birth weight, in contrast, appears to have a different developmental origin and is influenced more directly by maternal BMI.
The authors also emphasize that these findings are statistical inferences rather than definitive proof of causation. However, the results closely align with previous studies that used different analytical methods and genetic datasets, strengthening confidence in the overall conclusions.
Parental BMI was linked to obesity-related eating behaviors
The researchers also explored whether parental BMI was associated with children's eating behaviors at age 8. Higher maternal and paternal BMI were both linked to greater food responsiveness and emotional overeating in the offspring, and to lower emotional undereating. Fathers with higher BMI were also more likely to have children with lower satiety responsiveness and who ate more quickly. In addition, children with higher BMI tended to score higher for most obesity-related eating behaviors, with the exception of emotional undereating.
Together, these findings suggest that inherited genetic susceptibility may influence the risk of childhood obesity, in part through its effects on appetite and eating behaviors. However, the researchers caution that their structural equation model was not sufficiently powered to distinguish genetic from environmental influences on these behaviors, meaning this potential mechanism could not be confirmed.
Large family cohort strengthened the genetic analysis
The study examined a large prospective cohort that included non-twin siblings across generations and up to two children per parent. The model explored genetically explained relationships across parent-offspring pairs, as well as between cousins and between offspring and parental siblings. This ensured a more powerful model.
The study also has some limitations. The model assumes that a common environment is absent, a finding supported by previous studies and further analyses in the current study. It does not account for changes in gene expression in response to the environment, which could lead to an overestimation of genetic contributions. However, the authors did not expect this to be a significant factor.
The study sample included a Norwegian population from a high-income country with relatively high obesity prevalence, and the authors noted potential for selective recruitment and attrition, limiting its generalizability. Future studies in more diverse cohorts should replicate these findings.
Genetics appear to drive most childhood obesity risk
The results suggest that maternal BMI might substantially affect birth weight but has a much smaller effect on childhood BMI. The paternal link is likely to be still smaller. Thus, reducing parental BMI before pregnancy may be insufficient on its own to prevent childhood obesity in offspring through intrauterine mechanisms.
Long-term preventive strategies might also require non-stressful home environments that support healthy eating patterns and more physical activity, while recognizing genetic differences in obesity susceptibility between children.
However, strategies to reduce parental BMI before pregnancy might include durable changes in the family environment that contribute to reduced childhood obesity. Independent of child obesity risk, reduced prepregnancy maternal BMI is linked to better pregnancy outcomes.
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