More than 26 percent of American adults were obese as of 2009—compared with less than 20 percent in 2000, according to a new report from the U.S. Centers for Disease Control and Prevention. And the number of U.S. states with more than 30 percent of their population topping a body mass index (BMI) of 30 tripled between 2007 and 2009. With this accelerating epidemic, researchers are looking for clues beyond daily diet and exercise to explain our propensity for extra poundage—and many are finding evidence in the very first stages of life.

A growing number of analyses have found a convincing link among a heavier mother-to-be, increases in her baby's birth weight, and the child's later risk of obesity. In many past observational studies, however, basic genetics or environmental factors could be blamed for this association.

A new study of 513,501 mothers and 1,164,750 of their children born across 15 years aimed to take genetics out of the equation by assessing maternal and infant weight only for those women who had more than one child. "By making comparisons of two or more infants born to the same mother, we were able to factor out the role of genetics," says David Ludwig, an associate professor of pediatrics, director of the Obesity Program at Children's Hospital Boston and co-author of the new study.

Women who gained more than 24 kilograms during a pregnancy (which occurred in about 12 percent of pregnancies) added an average of 147.4 additional grams to their baby's birth weight than those who gained about 7.5 to 10 kilograms. In other terms, pregnant women who gained 22.5 kilograms had double the risk of having an infant with a high birth weight compared with those who only gained about nine kilograms. And every kilogram gained during pregnancy increased a baby's weight by about 9.5 grams, according to the analysis, which published online August 4 in The Lancet.

Being heavier at birth increases the odds that an individual will be overweight or obese as a child—as well as an adult. And the excess weight has been linked to a range of chronic conditions, including asthma, diabetes and metabolic syndrome (a group of metabolic risk factors).

Although previous studies had correlated high BMI moms with heavier babies, "the direct effects of excessive weight gain on the fetus have never been conclusively demonstrated," notes Ludwig, who worked on the study with collaborator Janet Currie, a professor of economics at Columbia University.

The importance of grams
The ill effects of undernourishment on fetal development have been well documented. A pregnant woman who does not get ample calories for her and her fetus increases the risk the baby will have stunted physical growth, poor cognitive development, and be more susceptible to diseases. The health risks of too many calories, however, are just beginning to come to light.

To be sure, a heavier fetus will tilt the pregnant mother's scale slightly, and the amount of weight typically put on my moms gaining too much during pregnancy far exceeds the additional ounces their babies typically take on.

Nevertheless, although 0.2 kilogram of additional baby fat might not sound like much, in the context of a three- to 3.5-kilogram infant, every 0.03 kilogram changes the odds ratio, according to Ludwig.

Other research indicates that infant birth weight is also heavily determined by a woman's weight even before she becomes pregnant. A study published in June in the European Journal of Pediatrics reported that being overweight or obese before getting pregnant meant that a mother's future child was 1.4 times more likely to be overweight or obese by age four. "It means preconception health screening and intervention for overweight and obese [women] is extremely important," says Panagiota Kitsantas, an assistant professor of biostatistics and epidemiology at George Mason University's Department of Health Administration and Policy and lead author of the June paper.

Although her investigation did not specifically look at women with more than one child and thus could have been colored by other genetic and environmental factors, Kitsantas says that the results from her work and The Lancet report are complementary. "Both studies pointed to one direction: mothers' body weight affected their offspring's weight."

Underlying changes
Extra birth weight might not be the only change many of these infants face. Excessive maternal weight during pregnancy is also likely changing the metabolic and hormonal environment of the developing fetus, Ludwig says.

Even if an infant has a few extra ounces due to a mother's excessive gestational weight gain, "the infant developed in a metabolically abnormal intrauterine environment," Ludwig explains.

Excessive caloric intake by a pregnant woman can stimulate the overgrowth of fetal tissues, change hormonal balances, alter metabolic pathways, "and perhaps even structures in the brain that regulate appetite and metabolism," he says. And those changes might stay with an individual for life.

Many adults have a difficult time losing weight and keeping it off, and if the body is predisposed to putting on the pounds, fighting obesity on both individual and societal levels will be even more challenging.

Researchers are still working to understand just how some of these pathways and hormones can influence disease risk, primarily through animal studies in the lab. And until more chemical links are found, a direct cause-and-effect relationship cannot be established, Kitsantas notes.

She applauds the new work, noting that Ludwig and colleagues used apt statistical models to try to avoid confounding effects and excluded subjects with other risk factors such as gestational diabetes or extremely high birth weight. Kitsantas is not entirely convinced, however, that genetics can be erased from the picture, and asserts that more lab work remains to be done to parse out nature, nurture and nutrition.

Prepregnancy health
Not every baby born on the heavy side will battle obesity or related chronic diseases. But, Ludwig points out, "on a population basis, [increased birth weight] is shifting risk upward."

The amount of weight pregnant women are putting on has been growing—as has their prepregnancy weight in the past few decades, Ludwig notes. Alongside that trend are signs that average birth weight is also headed upward.

"If we don't stop the vicious cycle at some point, we'll just keep going and going," Kitsantas says. If female babies are born more prone to obesity, the likelihood of their gaining too much weight before or during pregnancy increases, thus putting their offspring at greater risk.

Even though the specific mechanisms at work remain poorly understood and there is still not enough evidence to draw a cause-and-effect conclusion between maternal weight and a child's risk for obesity, Kitsantas says that is not reason enough to delay action. "We really have to jump in based on the findings we have to create specific interventions to fix the problem."

Ludwig acknowledges that the challenge of getting Americans to stay fit is great but says that changing the habits of mothers-to-be might be a little easier. "Women tend to be especially motivated during pregnancy because it's not just their health [that is] at stake—it's their children's," he notes. "Almost every mother instinctively wants to give their children a healthy start in life."

And, along with physical activity, food quality is just as important as quantity, he says. "The higher quality of diet consumed, the easier it is to maintain a health body weight," says Ludwig, who has been working on a new study comparing the effects of two different diets on maternal and infant health. "The best time to begin obesity prevention efforts for the next generation is actually prior to birth," he says.

Kitsantas extends that recommendation, suggesting that all women of childbearing age establish healthy lifestyle habits and healthy weights: "The sooner the better," she says.