• What is assessed?
• How is the measurement conducted?
• When is this method used?
• How are estimates of body composition derived?
• Strengths and limitations
• Populations
• Further considerations
• Resources required
• References

Rohrer’s Ponderal Index (PI) is an indication of a person’s weight relative to their height, and is used as a proxy measure of adiposity, similar to the Body Mass Index (BMI). PI is calculated as weight (kg) divided by cubed height (m³) (du V. Florey, 1970).

Compared to the more commonly used proxy for body fat BMI, (the person’s weight divided by the square of their height), PI better corrects for height, and therefore is a better estimate of adiposity, among newborn infants and also in the adolescent population (Peterson et al, 2017).

At birth, PI can be used to assess the pattern of fetal growth in small-for-gestational-age (SGA) infants (Armangil et al, 2011), by distinguishing symmetric from asymmetric intrauterine growth restriction, and quantifying the severity of asymmetry in growth-restricted neonates.

1. PI is calculated using measurements of height (or length) and weight.
2. The units of measurement (e.g. metric vs. imperial) must be clearly stated to avoid errors.

1. PI estimates proportionality.
2. At birth, PI is used in clinical settings, in nutritional surveys and large scale population studies as a screening tool to indicate whether a newborn infant is underweight, overweight, or a normal weight for their length. It is an indicator of fetal growth restraint: SGA is sometimes defined as a PI <3^rd percentile for gestational age and sex (Walther and Ramaekers, 1982).
3. PI has also been suggested to be a better proxy than BMI for adiposity among adolescents (Peterson et al, 2017), however is not yet widely used in this population.

If using the metric system, PI is calculated as body weight divided by the cube root of height, where weight is in kilograms and height in meters.

PI = Weight (kg) / [Height (m)]³

Other variables might be needed for the interpretation of this index such as sex, age and gestational age, however population reference data on PI are not widely available.

In infancy, like the other growth indices, PI can be independently assessed by the percentile point achieved by a child relative to the healthy children of that age and sex in the same population. Median (or the 50^th percentile) is regarded as a reference value, and 3^rd and 97^th percentiles as thresholds to indicate abnormally low or abnormally high values. A PI <10^th percentile reflects fetal malnutrition; a PI <3^rd percentile indicates severe fetal wasting. While a PI >90^th percentile was defined as neonatal overweight.

Strengths

1. PI is a important indicator of fetal malnutrition.
2. Easy to measure.
3. Non-invasive.
4. Among newborn infants and adolescents, PI is less correlated with height and length and is therefore a better proxy measure of adiposity than BMI.

Limitations

1. PI is only a proxy measure of adiposity, but does not directly assess the proportions of fat and fat-free mass.
2. In newborns, interpretation of PI values requires data on sex and gestational age and comparison to reference data.

Considerations relating to the use of ponderal index in specific populations are described in Table 1.

Table 1 Application of ponderal index in different populations.

Refer to section: practical considerations for objective anthropometry

Resources are dependent on the instruments/methods used to derive the raw data of height and weight.

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2. Fay RA, Dey PL, Saadie CM, Buhl JA, Gebski VJ. Ponderal index: a better definition of the 'at risk' group with intrauterine growth problems than birth-weight for gestational age in term infants. The Australian & New Zealand journal of obstetrics & gynaecology. 1991;31(1):17-9. Epub 1991/02/01.
3. Florey Cdu V. The use and interpretation of ponderal index and other weight-height ratios in epidemiological studies. Journal of chronic diseases. 1970;23(2):93-103. Epub 1970/08/01.
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10. Peterson CM, Su H, Thomas DM, Heo M, Golnabi AH, Pietrobelli A, et al. Tri-Ponderal Mass Index vs Body Mass Index in Estimating Body Fat During Adolescence. JAMA pediatrics. 2017;171(7):629-36. Epub 2017/05/16.
11. Sharma D, Shastri S, Sharma P. Intrauterine Growth Restriction: Antenatal and Postnatal Aspects. Clinical medicine insights Pediatrics. 2016;10:67-83. Epub 2016/07/22.
12. Walther FJ, Ramaekers LH. Neonatal morbidity of S.G.A. infants in relation to their nutritional status at birth. Acta paediatrica Scandinavica. 1982;71(3):437-40. Epub 1982/05/01.
13. http://www.medicalbiostatistics.com/childhealth.pdf