There are a number of factors that are related to the natural risk for scoliosis curve progression.
Being diagnosed with scoliosis at a younger age means there is a higher chance of curve progression. This is because the curve progresses most during the growing years in adolescence. The younger you are diagnosed, the more your bones have to grow thus, you are more likely to have your curve progress while you grow. The natural history of scoliosis has been studied in untreated scoliosis children and the following statistics are from a 1984 study published in Journal of Bone and Joint Surgery by Lonstein and Carlson. In this study for children 10 years or younger, a small curve (from 5-19 degrees) has a 45% chance of progressing however a medium curve (from 20-29 degrees) has nearly a 100% chance of progressing. This decreases as the child becomes older. For medium sized curves, at 11-12 years old there is a 61% chance of progression and by the time you are 15 or older, only a 16% chance of progressing. Later studies have demonstrated very similar data.
Skeletal maturity is another way to demonstrate how much growth a child has remaining. Certain growth plates close around certain ages for boys and girls. The Risser sign is a growth indicator in the pelvis that we see on scoliosis x-rays and refers to the amount of calcification of the human pelvis as a measure of skeletal maturity. It is graded on a scale from 0-5, with 5 being adult bone growth. In that same 1984 study for medium sized curves, a Risser sign of 0-1 shows 68% progression while those at grade 2-4 progressed by 23 percent. Again, the bigger the curve with more growth remaining means the curve will likely continue to progress.
Girls are much more likely to have scoliosis curves that progress to the point that treatment is required. For curves that are over 30 degrees, girls have a 10:1 ratio for progression over boys.
Certain curves have a greater chance of progressing. There are four major curve types that are named after their location. A thoracic curve is one of the most common patterns in idiopathic scoliosis with 90% occurring on the right side. A double major curve describes a right thoracic and a left lumbar curve. Both of these two curves are the most common curve patterns to progress. A thoracolumbar curve is also quite common with 80% on the right side and a lumbar curve is usually found on the left side.