Diane Peters

Especially in Baby’s early months, length and weight for age are two important measures of health and development. Sometimes your doctor will show you a line on a bar graph to indicate your little one’s growth, but most of the time you leave the office with a puzzling number called a percentile.

“This sounds like a mark on a report card,” says Cheryl Armistead, a community health nurse practising in Montreal. But if your child is at, for example, the 50th percentile for weight, it simply means that out of 100 typical children, 50 will be lighter and 50 heavier at the same age. It is not an indication that your baby or toddler is underperforming relative to one at the 70th percentile. A child under the third percentile may be considered underweight compared with the standard for her height and age; one over the 97th percentile may be considered overweight.

“So if your infant is under the third or over the 97th, you need to be asking if there’s a known cause for this,” says Armistead. While size alone doesn’t always indicate a problem, it can be a warning sign, bringing into play other questions and diagnostic tools.

A heavier child, for instance, could be at risk for becoming obese and all the health and social problems that may bring. So your physician may ask if the child is nursing or taking the bottle for comfort as well as for nourishment. Lightweights get even more scrutiny, as their diminutive size may suggest a feeding problem. Unexplained small size may also indicate a genetic problem or a growth disorder.

But common sense guides doctors in these cases. The child of a diabetic or an extremely tall mom is often larger. A premature infant, a twin or a baby of small parents is logically going to hit at the lower end of the chart. More worrisome than a large or small baby per se is one who falls off his normal growth trajectory. For example, a child dipping from the 50th percentile to the 30th will definitely get a doctor’s attention.

New Growth Standards

New World Health Organization growth charts issued in 2006 are based on data from 8,440 children in six different countries, many of whom had good prenatal and postnatal care and were both vaccinated and breastfed. They reflect the growth patterns of breastfed babies, who tend to plump up in the early months, but slim down by six months or so, meanwhile getting a little taller than their bottle-fed peers. So more infants may now initially be tagged underweight in the first three to six months, particularly if they are artificially fed. Over time, however, a greater number of babies and toddlers may fall into higher weight percentiles and shorter height percentiles. (Most physicians are still going by the old charts, based on U.S. bottle-fed babies.)

No matter which charts your doctor is using, always bear in mind that these are just one tool of many for assessing health. Chart-related concerns should lead to more questions and more diagnostic tools, so do not panic. “You should never take these measurements out of context,” says Armistead.