There is insufficient evidence to support the suggestion that a mother's body mass index during pregnancy has any long-term impact on the weight of her child in childhood and adolescence, finds a new study published in the journal PLOS Medicine. This finding has implications for interventions aimed at families to stop the obesity epidemic in its tracks.

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In contrast to previous studies, researchers have found that pregnancy BMI does not directly affect obesity in children and teenagers.

Being overweight or obese during pregnancy has been shown to cause the birth of heavier babies. However, it has not yet been established whether these babies are more overweight during childhood and adolescence as a consequence of their mother's body mass index (BMI) during pregnancy.

Previous research has shown that a mother's BMI is connected with increases in circulating nutrients, such as glucose, lipids, and fatty acids. Higher levels of these nutrients in gestation are associated with greater birth weight.

It has been suggested that exposure to nutrients that are linked to overweight and obesity in the uterus, such as glucose, may result in permanent changes to the child's appetite control, neuroendocrine functioning, and energy metabolism. These changes could, in turn, lead to the child being overweight or obese in later life.

The study authors point out that it is important to establish if a woman's BMI during pregnancy affects the BMI of the child through processes that occur in gestation; this mechanism might accelerate the obesity epidemic across generations.

If this were the case, it would emphasize a need for interventions to be considered before or during pregnancy to halt and prevent the obesity epidemic. Currently, obesity affects around 12.7 million children and adolescents in the United States.

Childhood obesity has immediate and long-term effects on health. Children and adolescents who are obese are more likely to have risk factors for heart disease that include high blood pressure and cholesterol. They are also at greater risk of developing prediabetes, bone and joint problems, and sleep apnea.

Children and teenagers who are obese have more chance of being obese into adulthood, which increases their risk of heart disease, stroke, type 2 diabetes, osteoarthritis, and several types of cancer.

Dr. Rebecca Richmond, senior research associate in epigenetic epidemiology at the University of Bristol, United Kingdom, and colleagues investigated whether a high BMI during pregnancy causes elevated levels of heaviness in children and teenagers.

Mother's genes, not pregnancy BMI, connected with obesity risk

The team evaluated the BMI and genetic information of 6,057 mother and child pairs from two birth studies - the Avon Longitudinal Study of Parents and Children (ALSPAC) and the Generation R study. The effect of BMI in pregnancy on the child's weight was measured several times between the ages of 7 and 18 years in ALSPAC, and at age 6 in the Generation R study.

Several other studies have suggested that BMI before or during early pregnancy may be associated with offspring obesity.

In the new study, the researcher's results also found a relationship between BMI before pregnancy and offspring BMI at all ages. However, the relationship was explained by the transmission of genes from mother to child that were associated with weight, not BMI.

Once the researchers integrated a genetic risk score into the analysis, they found almost no connection between BMI before pregnancy and the child's childhood BMI.

The findings suggest that there is no need for specific health interventions for women of childbearing age, but rather health interventions targeted at families. The study authors write:

"These findings suggest that public health interventions directed at all family members and at different stages of the life course are likely to be important and are potentially more likely to halt the obesity epidemic than a focus on maternal overweight and obesity status in pregnancy."

The authors continue to say that interventions aimed at the whole population - such as proposals for excess tax on foods that tend to contribute to obesity - may have a greater effect on obesity rates overall, than pinpointing BMI during pregnancy as the cause.

Limitations of the study include the fact that the data relied on self-reported pre-pregnancy BMI, and the study did not include data for weight gain during pregnancy. Also, power was limited for some of the sensitivity tests.

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