Table 2.

Normal bone development

Developmental period	Cardinal events	Major factors impacting bone	Other comments
Foetus	- Bone formation - Rapid longitudinal bone growth - Marked bone mineral accretion	- Gestational age - Body size - Most increase in bone mass and growth during third trimester of pregnancy	Entirely dependent on placental transfer of calcium and other minerals
Infant	- Rapid longitudinal bone growth - Marked bone mineral accretion	- Gestational age and body size at birth - Nutrition/breastfeeding, infections, drug and toxin exposures and activity level	Immediate shift to dependence on intestinal absorption, renal reabsorption and bone stores for calcium/minerals
Child	- On-going longitudinal growth and bone mineral accretion (slower pace)	- Nutrition, infections, drug and toxin exposures and activity level - BMI
Adolescent	- 26% of bone mass in 4-year period of peak height velocity - 60% of adult peak bone mass (PBM) is established	- Puberty - BMI - Age at pubertal onset as well as nutrition, infections, drug and toxin exposures, and activity level - Smoking - Alcohol use - Medroxyprogesterone and other drugs
Young Adult	- PBM achieved by age 20–25 years (varies by body site)	- BMI - Smoking - Alcohol use - Medroxyprogesterone and other drugs
Later Adulthood	- No net change in bone mass/density for many years (balanced bone formation and resorption) - Annual declines in BMD after fifth decade, especially with menopause	- Loss of bone with older age - Marked bone loss with menopause - Smoking - Alcohol use - Reduced physical activity - Nutrition

BMI=body mass index; BMD=bone mineral density; PBM=peak bone mass.