Those two little measurements can tell you a lot about a kid. Like if he's eating enough to keep up with his growth spurts - or if the food isn't quite keeping up with him.
While in Preah Vihear, I observed a follow-up for children under 5 years old who are enrolled in the nutrition program. This basically means that these kids fell in the under-nourished category when their height and weight were checked during screenings, as well as a measurement known as "MUAC" - mid-upper arm circumference. They get enrolled in the program, which entitles them to supplemental food basket vouchers every two weeks, bi-weekly follow-up measurements, and home visits and education by the staff.
Obviously, from the photos on my posts before, a lot of green stuff grows in Preah Vihear. These kids aren't living in deserts. And you will see a lot of photos of chubby babies under 6 months old. But when the kids start eating semi-solid foods and take in less healthy breast milk, they often lose their momentum on their growth spurts. Many times, the mothers simply don't know how to make nutritious meals on limited income or feed their children inappropriately. Healthy eating habits have to be learned; they aren't intuitive. And if kids don't learn healthy habits from their parents, they often start off life on the wrong foot.