考生们备考托福听力，除了扎实听力的基本功之外，也要对各种话题背景有所了解。托福听力背景知识一般会涉及到校园生活类对话和学术类讲座相关的场景词汇和场景套路。下面就将经常在新托福听力中出现的托福听力词汇背景知识进行总结，同学们将他们了解下来即可顺利的解决托福听力。
Mother's Pregnancy Weight Linked to Child's Obesity
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."