At each visit your pediatrician will measure your child's height and weight and mark them on a growth chart. Your doctor uses the chart to determine how your baby is growing compared with other children of the same age and gender. Percentiles are the lines on the growth chart; they tell you how many children are above or below your baby's measurement. For instance, if your 5-month-old boy is in the 70th percentile for height, it means that 70 percent of 5-month-old boys are shorter than he is, and 30 percent are taller.
Bigger is not necessarily better, however. How long or short or heavy an infant is at any point in time is less important than how that relates to where he was at a previous point. Small children are just as healthy as large ones as long as they are growing steadily -- which means they are progressing along a single percentile over time.
So don't get caught up in worrying too much about what percentile your child is in -- it's all relative. One of my patients, for example, has a mother who is barely 5 feet tall and a father who is well over 6 feet. The baby has been gaining and growing steadily along the fifth percentile. Her father's family is disappointed, while her mother's relatives are ecstatic that at least she is on the chart (many of them were not)!
When a child's weight gain or growth drops off its original curve, this could be a sign of a health problem, but there are many normal reasons that children "fall off the curve." Infants are born at a weight determined partly by their mother's nutrition and other influences in the womb. A newborn may be big because her mother is diabetic, for example, but the baby's weight percentile may drop off in the first few months of life as she veers toward her genetically determined size.
Perhaps because they are burning more calories, another time that weight may drop is at 9 to 12 months of age, when babies begin to pull up and start walking. The change, however, should not be dramatic, just a small dip in percentile. An illness can also cause a temporary drop from baby's usual percentile for weight.
Another important measurement taken at every visit is the head circumference, which is used to gauge baby's brain growth. Your child's brain grows the most during the first two years of life. Again, parents are often worried about whether their baby's head is too big or too small. Although either extreme can indicate a problem in rare instances -- a head could be growing too fast because something is creating pressure, for example -- usually it is simply due to genetics. By measuring the mom's and dad's head, I can figure out whether a big (or small) cranium is a family trait.