| Age in months | Length of body in metres | Monthly increase |
|---|---|---|
| 0 | 0.50 | 0 |
| 1 | 0.54 | 4 |
| 2 | 0.57 | 3 |
| 3 | 0.60 | 3 |
| 4 | 0.62 | 2 |
| 5 | 0.63 | 1 |
| 6 | 0.64 | 1 |
| 7 | 0.65 | 1 |
| 8 | 0.66 | 1 |
| 9 | 0.67 | 1 |
| 10 | 0.68 | 1 |
| 11 | 0.69 | 1 |
| 12 | 0.70 | 1 |
The same facts appear from the combination picture given by Stratz, showing an infant's skeleton at four-month intervals from birth to the end of the first year.
During the second year of life, the increase in stature is about one-half that of the preceding year, that is, about 10 cm. (4 in.), so that at the end of the second year the child attains a height of about 80 cm. (31½ in.). After this, the annual increase diminishes in intensity (see "Figures of the increase of stature according to Quétélet and other authors," in the technical part, Chapter VII), as is shown by the horizontal dotted lines, which, starting from a vertical line at points corresponding to the height of various statures, represent by the intervals of space between them the successive growth from year to year.
This increase is not regular, but proceeds by periodic impulses that in early childhood seem to recur at intervals of three years.
Thus for example the increase
- between 0- 3 years of age is successively 20, 10, 6 cm.
- between 3- 6 years of age is successively 7, 6, 5 cm.
- between 6- 9 years of age is successively 7, 6, 5 cm.
- between 9-12 years of age is successively 6, 4, 3 cm.
Accordingly we have a triennial rhythm, decreasing throughout the whole period of childhood; the maximum increase is in the first triennium, the second and third periods of three years correspond exactly, while the last period shows a lowered rate of increase.
At this point the period of approaching puberty begins (13 years for boys), after which the rate of increase becomes more rapid than it had been during the second or third period, attaining its maximum during the years 13-15; to be specific, the rate from 13 to 18 is successively 4, 8, 7, 5, 6, 3 cm.
When the period of puberty is ended (18 years), the rate of growth is much slower; in fact, during the two following years (18 to 20) it hardly attains one centimetre.
Nevertheless, the stature continues to increase up to the twenty-fifth year; according to Quétélet's figures, the average male stature at the age of eighteen is 1.70 m. (in Belgium) and at twenty-one it is 1.72 m.