"We found that, compared with children born from mothers of normal weight, those whose mothers were overweight or obese during pregnancy had up to 20 to 30 percent higher odds of asthma," said lead researcher Dr. Erick Forno, an assistant professor of pediatrics at Children's Hospital of Pittsburgh.
Forno's team also found that excess weight gain during pregnancy was associated with about a 16 percent increased risk of asthma in the children. "These results included studies that evaluated asthma at different time points in childhood, from a little over a year of age all the way to 16 years of age," Forno noted.
Although this review of more than a dozen previously published studies found an association between a mother's weight in pregnancy and her child's risk of asthma, it was not able to prove that a mother's weight is a direct cause of childhood asthma.
How a mother's weight might contribute to an increased risk of asthma in her children isn't clear. "It is important to clarify that the studies we analyzed did not directly evaluate the mechanisms involved in this association, so we don't know exactly how this link works," Forno said.
Several factors may be involved, he noted. "We know for example that obesity sometimes leads to inflammation that can contribute to diabetes or heart disease, and perhaps this inflammation in the mother somehow affects the developing lungs and airways in the baby," Forno said.
Or perhaps certain nutrients that mothers with healthier diets ingest may protect her offspring from asthma, he said. "Another mechanism may be that certain factors in the genetic make-up of the mother predisposes both to obesity (in herself) and to asthma (in her child). Most likely there is a combination of all these playing a role," Forno suggested.
Forno noted that it is important that all women of reproductive age maintain a healthy weight, especially if they are trying to get pregnant.
"It is also important to have adequate weight management and healthy nutrition during pregnancy, because excessive weight gain [during pregnancy] also increases the risk of childhood asthma. This is of course in addition to all the other benefits of a healthy weight and diet," he said.
The report appears in the August print edition of the journal Pediatrics.
Previous studies have found that obesity leads to an increased risk of asthma. And, among children with asthma, obesity leads to increased severity of the disease. There has also been increasing interest in the early origins of childhood asthma, Forno said.
To see how a mother's weight might be linked to the development of asthma in her children, Forno and his colleagues reviewed 14 previously published studies that included over 100,000 mother-child pairs.
The risk of a child having asthma was 36 percent higher for mothers who were obese during pregnancy compared to normal-weight expectant mothers. The study authors suggest that, for women who do not have a history of asthma, the effect of obesity during pregnancy may be an even stronger influence on whether or not a child develops asthma.
Although there appeared to be a slight association between an overweight mom-to-be and asthma in her offspring, the association wasn't statistically significant, according to the study.
Dr. David Mendez, a neonatologist at Miami Children's Hospital, said that this study asks a question that only additional research can answer. Such research would need to take into account the mother's history of asthma and the baby's exposure to cigarette smoke, among other factors, he said.
"This kind of study would take years to do because you have to wait for children to grow up," said Mendez, who was not involved with the study.
"We already know maternal obesity and increased weight during pregnancy are bad for the mother and bad for the baby," he added. Being overweight can result in low-birth-weight infants, preterm delivery and cesarean delivery.
"There are short-term reasons why mothers shouldn't gain too much weight during pregnancy, and now it appears that there may also be long-term benefits to doing that as well," Mendez said.
For more about asthma in children, visit the American Lung Association.