Handy rules of thumb for predicting adult height from parents' height, or
from height at age two years, may give you a general indicator of final
height, within 2" either way, and many predictions will turn out correct,
given that you have 4" leeway.  However, HOW children get to their final
height will vary greatly, thus leading to concerns that either the child is
growing too slowly, or growing too quickly.

Height is a physical characteristic that influenced by many different genes,
as well as many different environmental factors.  There are genes for:

rate of growth in early childhood (fast, slow, in-between)
mid-childhood growth spurt (present or absent, marked or slight)
age at onset of puberty (later puberty means taller final height, others
things equal)
intensity of adolescent growth spurt (hardly noticeable to very intense spurt)
length of adolescent growth spurt (just a few months or several years)

Growth in height stops within two years of adolescence, for most people.  So
a girl who goes through puberty at age 10 will, on average, be much shorter
than a girl who goes through puberty at age 15.  However, a girl who has a
fast rate of growth in childhood and an early but intense adolescent growth
spurt can end up 5'8" tall, while her sister can have a slow rate of growth
in childhood and a late growth spurt and also end up 5'8" tall.

Environmental factors that affect growth in height include childhood
nutrition, childhood disease levels, and emotional stress.

Any technique that uses mid-parental height as part of the formula will only
work if the parents had good nutrition and physical and emotional health
during childhood.  Any parents coming from backgrounds of deprivation
(whether due to malnutrition, disease, or emotional stress such as war) who
raised their children under better conditions, will have children who are
taller than predicted.  This is called "positive secular trend" and is quite
easy to observe in the US by going to any mall and looking at the heights of
multiple generations of immigrant families.

Kathy Dettwyler

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