The term Osteopenia is defined as a bone mineral density between 1 and 2.5 standard deviations below the young adult mean value. Osteoporosis is defined as a bone mineral density more than 2.5 standard deviations below the young adult mean normal.
Bone density has a genetic component Three distinct genetic factors contribute to fracture risk.
First, peak bone mass occurs between ages 30 and 35. Second, after about age 35, bone mass decreases over time and fracture risk increases. The decrease of bone density is exacerbated by the loss of estrogen after menopause in women, a genetically determined event. Third, the genetically determined length of the femoral neck correlates with fracture risk, people with a longer femoral neck having an increased fracture risk. Daughters of women with osteoporosis have been noted to have reduced bone mass in the spine and probably also in the femoral neck that would put them at increased fracture risk.