First: a gap alone does not mean disease
Many parents react with worry when they receive a bone age report. In fact, some difference between bone age and chronological age is common in healthy children — differences of around ±1 year are generally considered normal. What carries meaning is the size of the gap together with the child's growth curve, pubertal signs and family height.
This article is for information and does not replace medical assessment. Always interpret your report with the physician following your child.
If bone age is advanced (higher than chronological age)
Advanced bone age means the skeleton is maturing faster than expected. Such a child may be taller than peers right now, but because the growth plates will also fuse earlier, the growth period is shorter. Being tall now does not necessarily mean a tall adult height.
Possible causes include:
- Precocious puberty, or puberty starting at the early end of normal
- Obesity — the hormonal effects of body fat can accelerate skeletal maturation
- Excess of certain hormones (thyroid, adrenal or sex hormones, for example)
- A familial/constitutional tendency to rapid growth
If bone age is delayed (lower than chronological age)
Delayed bone age means the skeleton is maturing more slowly. The child may be shorter than peers now, but the growth plates stay open longer, so the growth period is longer too. In many cases this is good news.
The most common situation is constitutional delay of growth and puberty: the child grows slowly but healthily, enters puberty late, and eventually reaches a height close to their genetic potential. A similar family history is often present. Other possible causes:
- Growth hormone deficiency
- Hypothyroidism
- Chronic illness, malabsorption such as coeliac disease, inadequate nutrition
- Long-term steroid use
- Chromosomal and skeletal development disorders
Bone age and adult height prediction
The most-asked use of bone age is adult height prediction. The logic: bone age shows how much of the child's growth potential has already been used. Current height, bone age and sex are combined into an estimate (classically the Bayley-Pinneau and TW methods).
These predictions carry a wide margin of error, however — typically on the order of several centimetres in the literature. Treat the estimate as a range, not a promise. In children whose bone age is markedly advanced or delayed, the margin widens further.
When to see a doctor
The following warrant a paediatric endocrinology assessment regardless of the bone age result:
- Height markedly behind peers, or a drop across growth-curve percentiles
- Less-than-expected growth per year (slowing growth velocity)
- Signs of puberty before age 8 in girls or 9 in boys
- No pubertal signs at all by age 13 in girls or 14 in boys
- More than 2 years' difference between bone age and chronological age