Pubertal development

Superimposed on the more gradual age-related changes, more rapid changes in anthropometric measurements occur during sexual development. For example, during the adolescent growth spurt, the highest rate of weight gain follows the highest rate of height gain ⁴². This leads to an acceleration in the rise of BMI shortly after reaching the peak height velocity, and this rise is more related to pubertal development than chronologic age ^{42, 43}. Moreover, sexual development occurs at different ages in different populations ⁴⁴. As a result, differences between study populations and reference populations in sexual maturation can confound comparisons. Therefore, the comparison of a pre-pubertal child in a study population to post-pubertal individuals of the same age in a reference population would be invalid because the normal BMI for these two persons would be quite different. Chronic undernutrition can also delay sexual maturity and the adolescent growth spurt ^{4, 5, 44}. This delay can exaggerate differences in age of sexual maturation and the growth spurt between undernourished survey populations and well-nourished reference populations ⁴⁵.

Associations are also found between sexual development and skinfold thicknesses and MUAC. The onset of puberty changes the rate of subcutaneous fat deposition and fat distribution, as measured by skinfold thickness,^46-49 and these changes occur independently of chronologic age ⁵⁰. Similarly, MUAC changes with onset of puberty ^{47, 49}.

Adjustment for differences between the survey and reference populations, as recommended by WHO, requires the collection of data on the age at which certain landmarks of sexual maturation occur in the survey population. Each subject must be assessedfor pubertal changes; however, in practice, judging Tanner breast or genital stages during field surveys is very difficult. Health workers require extensive training and, in most field situations, exposing breasts and genitals of adolescents is not culturally acceptable. The single maturational landmark which might easily be collected in the field is age of menarche. In all populations where adolescents know their ages, age at menarche can be recalled by girls and women, and has been measured successfully in many surveys ^{2, 4, 43, 49, 51-53}. However, the situation is very different for boys, for whom no such easily obtained landmark exists. Self-assessment of genital and breast stage, using photographs or line drawings, has been validated in some countries, but has not been evaluated in developing countries or in a variety of cultures ^54-56. (See figure 2 for an example of drawings used in self-assessment.)

Figure 2 Examples of line drawings of various Tanner stages of sexual development