Michelle Obama said that she is paying more attention to her daughters body fat measurement.
Obama said she was surprised to learn that her daughters' body mass index, or BMI, numbers were "creeping upwards," she wrote on yahoo.com's website.
"I didn't really know what BMI was," she said.
"I certainly didn't know that even a small increase in BMI can have serious consequences for a child's health," she added, recommending that all parents inform themselves about the vital weight statistic.
President Barack Obama and the first lady are the parents of two girls, Malia, 12, and Sasha, 9.
The BMI takes height and weight into consideration when calculating body fat, and is considered by doctors and fitness experts to be a more reliable indicator of obesity than weight alone.
Michelle Obama has targeted childhood obesity as her signature cause, at a time when one in three American children is obese or overweight due to a lack of exercise and a diet loaded with fat and sugar.
Overweight children are believed to be more likely than their normal-weight peers to grow up to be obese adults at greater risk of high blood pressure, high cholesterol and type 2 diabetes.