The Percentile Puzzle

Between the APGAR test, administered minutes after a baby is born, and a child’s first report card, there isn’t much to officially gauge a child by. Perhaps that’s why many parents latch on so firmly to percentile points. “Ninety-fifth percentile? What a big, healthy boy I have!” “Fortieth percentile! I always knew my daughter would be petite!” Truth is, the numbers alone are no more an indicator of a child’s potential size than an APGAR test is in predicting a child’s SAT scores.

“Percentiles are a tool used to monitor a child’s growth,” explains Dr. Sarah Hambrook of Shoreview Pediatrics in Milwaukee. “They don’t matter nearly as much as the pattern of a child’s growth.”

From birth to age one, four traits are measured: height, weight, height for weight and head circumference. Those results are compared to national averages to determine the percentile in which the baby scores. Boys are compared with other boys, girls with other girls. For instance, a baby boy, 6 months old and weighing 16 pounds, would place in the 95th percentile – meaning he is heavier than 95 percent of the average population of 6-month-old boys. Similarly, a 6-month-old boy who weighs eight pounds would be in the 25th percentile, meaning he is heavier than 25 percent of his peers, but weighs less than 75 percent of that same group. It’s a snapshot of a measurement at a particular moment in time. Not until these results are plotted on a graph and a trend is established do the numbers take on any meaning.

“If a baby is in the 10th percentile but stays along that line, that’s fine,” Dr. Hambrook says. The key is that the baby has consistent growth in all four areas. If one area were to suddenly deviate from the projected trajectory, it would signal the need to look for a potential problem.

Causes for concern, reasons for reassurance

“We want children to stay in a certain range,” explains Dr. Florence Sandmann of Westbrook Pediatrics in Brookfield, Wisconsin, which is affiliated with Children’s Medical Group. If a child drops or increases to a significantly different percentile in either weight, height or head circumference, a doctor would then know that extenuating circumstances may be at work. These could include a chronic disease, such as childhood diabetes or arthritis, or a thyroid or pituitary gland disorder.

“Overall, percentiles help parents and physicians know about a child’s nutritional status and can be an early predictor of other factors,” Dr. Sandmann says. “They are an objective account of the rate of a child’s growth. The can give us comfort, or alert us sooner rather than later.”

Sometimes, children look healthy even when there is a problem. “Sometimes there is no outward sign, such as with thyroid problems or hormone deficiencies,” says Sandmann. “Seeing a change on the chart means you don’t let it go just a while longer.”

The charts can also reassure parents that nothing is wrong with their child. “A lot of children become picky eaters after age one,” says Sandmann. “Parents will be concerned that their child has gained only two pounds. But we can point to the charts and say, ‘See, she’s still on the same track and placing at the same percentile.’”

Natural shifts

There comes a point in the latter half of the first year when an infant’s percentile plots do change, and it is completely natural. While the early months of growth reflect the baby’s in-utero or gestational growth, months six through twelve begin to reflect genetics.

Dr. Sandmann used the example of The Six-Foot-Five-Inch Parents and The Five-Foot Parents. Both could have a newborn weighing eight pounds and measuring 19 inches long. “Between six to twelve months, the two babies will start following heredity. The Six-Foot-Five-Inch Parents will see their baby maintain that 95th percentile. The baby of the Five-Foot Parents will start meandering down the chart to the 75th percentile or 50th percentile due to family genetics.”

“You can only work with what you’re given,” Dr. Hambrook adds. Some babies, of course, can be larger than their genetic predisposition. Children of first-generation immigrants, for instance, will be larger than their parents due to better nutrition. Preemies and children born with Down syndrome will also reflect growth trends that differ from their parents. Special growth charts provide a more accurate representation for these groups by gauging their growth in comparison to their peers.

Bigger babies bring updated charts

Growth charts have been in use by pediatricians since the 1950s and ’60s. Dr. Hambrook explained that major replotting was done in the ’90s when it was realized that the data gathered between 1953 and 1975 came from mostly Caucasian, bottle-fed babies born to middle income families. Another update to the charts was completed in 1999, reflecting better overall nutrition among the American population in conjunction with an increase in mothers who chose to breast feed.

“For a long time we used the charts from the ’70s,” Dr. Sandmann says. “But the population as a whole is getting larger thanks to better diets, better lifestyles and less disease. Now, what was the 75th percentile is considered average.”

Fixate on personality, not percentile

“Parents seem to fixate on percentiles a lot,” Dr. Hambrook says. “I think a lot of it is they are so nervous they’ll do something wrong. Percentiles give them a measure of reassurance. And newborns don’t do a whole lot, so it is one way to differentiate them.”

Katherine Rummler’s parents, Stephanie and Kurt, knew all about percentiles by the time she came along. Preceded by two brothers, Daniel, 5, and Ben, 2, Katherine has charted, “on the low end, especially compared to the boys,” says Stephanie Rummler. Daniel, in particular, gave his mom and dad reason to relentlessly watch the percentiles for his head circumference. “He kind of had a Charlie Brown head,” Rummler explains. Larger than “normal,” it was a source of concern for the then first-time parents. “Our doctor reassured us and told us not to worry about it, that Daniel would grow into it.” He did. That same doctor is still reassuring Stephanie and Kurt where Katherine’s growth is concerned. “Our doctor always says she’s progressing nicely,” says Rummler. “She’s been in the 50th percentile the whole way.”

Wendy Neuok admits to being fixated on the percentiles with her first-born, Monica. Born weighing an average seven pounds, 13 ounces, Monica grew very quickly. By her two-month check up, she plotted a solid 95th percentile. “We focused on that at first,” Neuok says. “But she was proportionate in all the measurements, and no one area was excelling. She was equally tall as heavy, and the two didn’t separate. That was enough to calm me down. She’s been at the 100th percentile ever since.”

When her second child, Paul, was born in March of this year, he was smaller in comparison to his sister. “At his last check up, he was at the 50th percentile all the way around,” Neuok describes. “The shape of the curve (on the growth chart) is normal, no valleys or peaks. I thought, ‘Oh, that’s what a normal baby looks like.’ Each child is their own being, and they grow in their own way.”