Preconception Metabolic Bariatric Surgery and Offspring Infant Growth Trajectories and Body Size from Birth to Age at School Entry: The POSIT Study
Although preconception metabolic bariatric surgery improves maternal health and reduces the risk of large-for-gestational-age births, it does not lower the risk of childhood obesity and may even increase the likelihood of severe obesity by age five compared to controls matched by pre-pregnancy BMI.
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For decades, doctors have known that a mother's weight before she becomes pregnant shapes the health of her child. When a woman carries extra weight, her baby is more likely to be born too large, which can lead to complications during birth and a higher chance of obesity later in life. Conversely, if a mother is underweight, her baby might be born too small. Because of these risks, medical guidelines now suggest that women with severe obesity try to lose weight before conceiving. For many, the most effective way to achieve this is through metabolic bariatric surgery, a procedure that reshapes the stomach to help people lose a significant amount of weight. It is a powerful tool that has helped thousands of women improve their metabolic health and avoid dangerous pregnancy complications like gestational diabetes. But a crucial question has remained unanswered: does this weight loss actually protect the next generation? If a mother loses weight before pregnancy, does her child grow up with a lower risk of obesity, or does the child's body size follow a different path entirely?
To find the answer, researchers at Kaiser Permanente and several other institutions launched a massive study called POSIT. They looked at more than 1,300 children born to women who had undergone metabolic bariatric surgery before becoming pregnant. To understand what was happening, the team compared these children to two different groups of other children. The first group was matched to the mothers' weight before they had the surgery, representing what would have happened if the mothers had never lost the weight. The second group was matched to the mothers' weight after the surgery but before pregnancy, representing a fair comparison between a mother who lost weight and a mother who naturally stayed at that lower weight. By tracking these children from birth until they were five years old, the researchers could see if the surgery changed the child's growth story.
The results confirmed what was already known about the immediate effects of the surgery on the womb. Children born to mothers who had the surgery were much more likely to be small for their gestational age, meaning they were smaller than expected for how long they had been in the womb. At the same time, they were far less likely to be large for their gestational age. This makes sense, as the surgery changes how the mother's body absorbs nutrients and alters the environment inside the uterus. However, the story changed once the babies were born. Instead of staying small or growing into leaner children, the infants born to mothers who had the surgery quickly caught up in size. By the time they were one year old, their growth trajectories looked almost identical to the children of mothers who had never had the surgery but were just as heavy before pregnancy.
As the children grew older, the pattern became even clearer. By the time the children reached five years of age, those born to mothers who had the surgery had the same risk of severe obesity as the children of mothers who were still carrying their pre-surgery weight. In fact, about 16 percent of the children in the surgery group were classified as having severe obesity by age five, a rate nearly three times higher than the children of mothers who had naturally maintained a lower weight before pregnancy. The researchers found that the type of surgery, whether it was a sleeve gastrectomy or a gastric bypass, did not change this outcome. Nor did the amount of time between the surgery and the pregnancy; even if the mother had lost weight years before conceiving, her child's risk of obesity remained high.
These findings suggest that while metabolic bariatric surgery is excellent for improving a mother's own health and reducing certain pregnancy risks, it does not seem to reset the genetic or biological programming that leads to childhood obesity. The study indicates that a child's weight is driven less by the mother's weight at the exact moment of conception and more by the deep-seated factors that caused her severe obesity in the first place. It is as if the mother's history of high weight leaves a lasting mark on the child's development that weight loss alone cannot erase. The researchers conclude that while losing weight before pregnancy is vital for the mother's safety, families and doctors should not expect it to automatically prevent obesity in the child. Instead, the focus may need to shift toward understanding the complex mix of genetics and environment that shapes a child's growth, regardless of how much weight the mother lost before the baby arrived.
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