My baby daugther was at the pediatrician today and we were told that her length was at 75 percentile but her weight was at 50 percentile.

Now normally precentiles, like on a standardized test, indicate that your score was better than X% of other test takers.

Is it the same for these infant scores or are they comparison to an average for that age? In other words, is she longer than 75% of all babies at that age or is she 75% of the average length of all babies at that age? Since she is quite small (in my opinion), that latter seems more likely.

Sounds like she’s taller than 75% of babies her age. If she’s under a year, a couple inches may be the difference between the 75th or the 40th percentile.

My toddler is similar, 85th in height, 50th percentile in weight, 90th percentile in head measurement. If he stays tall, thin and smart, I’ll steer him toward quarterback and/or point guard.

(I know head measurement and IQ are not directly correlated, but this kid picks up new concepts, words, numbers, problem solving like a little genius!)

Your daughter is, according to the chart, longer than 75% of babies her age, and shorter than 25%. She is heavier than 50% of babies her age and lighter than 50%.

Assuming, of course, the measurements are accurate, the doc charted them properly, etc. Since these things can’t be guaranteed, you want to look at trends over time, rather than a single measurement.

Infant statistics are supposed to mean the same thing as any other statistic, but there is the problem that a doctor may not have done well in his/her statistics courses.

The difference is the subtle linguistic one of being “at the 75th percentile” or being “at 75 percent.”
Being at the 75th percentile is supposed to mean that she is longer than (or as long as) 75% of other babies her age.

Being 75% would mean that she is 3/4 of the average length of babies her age.

Most doctors use the percentile, and use it correctly, but it is always possible the doctor misspoke.

In any case, the figures you quoted are nothing to worry about nor to be proud of. It does not mean she’s tall and skinny. It does not mean she will be tall and skinny. A matter of a fraction of an inch could be enough to put her at 75th percentile.

In regards to CK’s comment about 75th percentile vs. at 75 percent, I don’t think it is an issue of misspeaking (sp?). I understood this to be a standard metric and have heard about it from relatives about other infants. In other words, this is a standard and is either percentile or percent, not something where one doctor may report percentile and the other percent.

The percentages for infant size are like the percentiles for standardized tests, with a couple of important differences.

On a standardized test, the statistics reflect the entire population. That is, the adminstrators know the score from every single test. So when they tell you your score is at the 75 percentile, they mean business.

With children, there is no central databank that collects the sizes of every single child every time he/she is measured. The percentiles are based on a statistical sample, not the entire population. So if your baby is at the 75th percentile in weight, she weighs more than 75% of all babies estimated by the sample. That is why you can have cases like my son, who at 1 year old was at the 110th percentile. He was off the charts for the sample, but obviously the range of that day’s population is greater than that of the sample.

For a standardized test, counting all scores, it is impossible to be above the 100th percentile. With sampling it happens.

Also, I meant to tell you that early percentiles are not always a good predictor of future size. My son was born big (over 9 lbs) but is now very small for his age.

I’m not sure, but I thought I heard that the current charts (they are revised periodically) are based on data from a group of caucasian (bottle-fed only?) babies. If so, the charts could be less accurate for other races, etc. Again, my recollection of this is vague…anyone know for sure? Cite?

I don’t know how old your baby is, but we tend, given good nutrition and a healthy lifestyle, to birth the largest babies that our bodies can handle, for the simple reason that bigger babies have a better start on life. Their eventual size cannot be predicted from their birth weight.

My first was 8 1/2 lbs, and the other three were each just a few ounces, one way or the other, from 9 1/2 lbs. My second, though starting out at the 95th percentile, leveled out right around the middle. My third has turned out to be quite petite, barely hitting 25th on either height or weight since she was 18 months old. My youngest, on the other hand, may well hit 6 feet, if doubling her height at two years of age is really a good predictor.

And, yes, the charts are built on numbers derived from a particular subset that, as I understand it, does include mostly American, middle-class, bottle-fed babies these days. Those numbers may not indicate true averages/means for another particular subset.