What does your doctor mean when she tells you your child is in the "fiftieth percentile" for weight on the growth chart? This is another one of the terms we physicians sling about and forget to define.

What it means is this: of one hundred average babies born on your child's birthday, your child's weight or height ranks fiftieth. Forty nine of the kids are smaller, and 50 are heavier or taller.

We generally expect children to fall between the 5th and the 95th percentiles, but if your child falls outside of this range, it is not necessarily a sign of anything wrong. These outer boundaries of the expected range of variation are simply statistically derived markers for the generally expected, not the totally normal.

Physicians who care for children are usually happy if the percentile ranks for height and weight are fairly consistent: "50th percentile for height, 40th percentile for weight - that's just fine, Mrs. Jones." We are also carefully watching the long term trend - we want the child to stay about the same on the percentile ranking over time. Unexplained changes, especially when a child "falls off the curve" can be a sign of underlying medical illness. See also growth.
