Results of the study, which included more than 3,000 children, suggest that a child is far more likely to be overweight at a very young age ? at 2 or 3 years old ? if his mother was overweight or obese before she became pregnant. A child is also at greater risk of becoming overweight if he is born to a black or Hispanic mother, or to a mother who smoked during her pregnancy.
And there's a good chance that an overweight child will stay overweight for the rest of his or her life.
"Weight persists with time, so a child who is overweight by her second birthday is more likely to be overweight at a later age," said Pamela Salsberry, the study's lead author and an associate professor of nursing at Ohio State University. "Prevention of childhood obesity needs to begin before a woman ever gets pregnant."
Salsberry conducted the study with Patricia Reagan, a professor of economics at Ohio State. Their study appears in the December issue of the journal Pediatrics.
The researchers analyzed the data for 3,022 children included in the National Longitudinal Survey of Youth's (NLSY) Child-Mother file. The NLSY collected height and weight information at multiple points in time. In this study, children were weighed when they were roughly ages 3, 5 and 7. The survey also gathered information on each child's race and ethnicity, and asked each mother to recall her pre-pregnancy weight, if she had smoked while pregnant and if she had breast-fed her child.
Children were considered overweight if their body mass index (BMI) was greater than or equal to the 95th percentile for their age and gender. BMI is a measurement that relates weight to height. A child in the 95th percentile for his weight is heavier than 95 percent of the children his age.
A mother's weight within a month or two before she became pregnant had the greatest impact on a child's weight at all three weight measurement points.
If a woman was overweight before she became pregnant, her child was as much as three times more likely to be overweight by age 7 compared to a child whose mother was not overweight or obese.
There was a significant relationship between a mother's weight prior to pregnancy and her child's weight. The risk that a child would be overweight at a young age increased with the degree of the mother's obesity.
At each weight measurement point, about 4 to 6 percent more black and Hispanic children were overweight than white children. However, the percentage of all children who were overweight, regardless of race or ethnicity, decreased with age.
"Some children lose extra body weight and become leaner as they grow," Salsberry said.
Children of mothers who smoked during pregnancy were more likely to be heavy at all three weight measurement points.
"Obviously smoking during pregnancy causes a host of serious problems, but this finding adds to the growing body of evidence that suggests that smoking during pregnancy may be a key risk factor that increases a child's chances of being overweight," Salsberry said.
Breast feeding had a slight effect on weight at each measurement: As much as 5 percent fewer children who were breast-fed were also overweight, compared to bottle-fed babies.
The researchers also looked at other factors that may affect a child's weight, such as the age of the mother when she gave birth, the child's gender and whether or not the mother was married. None of these factors had the same degree of effect on childhood weight as a mother's weight prior to pregnancy, race, ethnicity or smoking.
Two out of three children who were overweight at their final weighing were also overweight during at least one prior weighing. Three out of four children who were at a normal weight at the final weighing had always been at a normal weight.
"A child's weight at 3 years is a good prediction of what his weight will be at age 5, and so on," Salsberry said. "Weight states tend to persist over time.
"Obesity continues to rise in adults," she said. "And that risk has increased in children, too. Interventions should begin immediately for children who are already overweight at these young ages."
In related work, Salsberry has also looked at the weights of teenagers and young adults.
"We often see overweight 12- and 13-year-olds whose parents are also overweight or obese ," she said. "It's the same thing with young adults around 20 and 21. There is evidence suggesting that the development of obesity in these age groups may be related to dietary habits very early in life."