Watching children while they eat for cues about when they are full and encouraging them to drink plenty of water may help control their weight, according to two studies in the journal Pediatrics.

Obesity rates among U.S. children have doubled in the last 20 years, and almost a third of American children are either overweight or obese. The epidemic of obesity is linked to a host of health problems such as higher risks for heart disease, diabetes and cancer.

Researchers at Harvard University and Children's Hospital, Boston, divided 559 children into four groups based on measurements of their weights and heights at birth, six months and age 3. Nearly one of 10 children were obese at 3.

Those heaviest at birth and those gaining the most weight through age 6 months together had a 40 percent higher probability of being obese at age 3.

"(This) suggests that it is rapid weight gain in infancy that puts children at risk" more than heavy birth weight, Elsie Taveras and fellow researchers wrote.

Obesity at age 3 does not necessarily translate into obesity later in childhood or adulthood, but it raises the risks, they said.

The Harvard researchers said it was important to educate parents and other caregivers to improve infant diets and to help them recognize when children are full.

A second study in the journal by Rebecca Muckelbauer and colleagues at the Research Institute of Child Nutrition in Dortmund, Germany, found an easy way to keep children from gaining too much weight was to encourage them to drink more water.

Water fountains were installed in schools in poor areas of Dortmund and Essen, Germany, and 1,641 second- and third-graders were urged to drink more water. They drank the equivalent of an extra glass of water a day compared to children not given the encouragement.

Those who drank more water lowered their risk of becoming overweight by 31 percent.

How do you, as a parent, ensure that your child is eating correctly? Share your secrets in the box below.