The bone age of the hand and wrist will be assessed with X-rays in more than 20,000 healthy children age 3-18 years old recruited in 28 sites from 14 province.The bone mineral density (BMD) will be measured as well.

As part of the "National Nutrition and Health Systematic Survey for 0-18 years old Children and Utilization of Survey Findings "(ChiCTR1900022825), more than 20,000 children age 3-18 years old was recruited from 28 sites of 14 provinces within the framework of the ChiCTR190002282 study protocol. The population of this study was proportional to the ChiCTR190002282 study. The X-ray of no-dominant hand and wrist was taken with a mobile X-ray unit with shielding. A step wedge was imaged along the side of the hand at the same time for bone mineral density measurement. The bone age of the hand and wrist was assessed by trained MSK radiologists according to Chinese standard. The bone mineral density of the wrist was measured with radioabsorptiometry (RA). The demographic parameters were collected by the study of ChiCTR190002282 protocol.

Inclusion Criteria:

• participants of ChiCTR1900022825 study
• healthy children age 3 to 18 years old.
• willing to participate this study
• Consent by parents

Exclusion Criteria:

• no consent by parents
• under 3 years old

As part of the "National Nutrition and Health Systematic Survey for 0-18 years old Children and Utilization of Survey Findings" (ChiCTR1900022825). This population is the subset of the ChiCTR1900022825 study, age 3-18 years old, sample proportion to the general study population. General healthy children age over 3 years old up to 18 years old from the kindergarden, primary school, middle school and high school.