How tall will your child be? It’s a question that has vexed parents for centuries.

Now, say Canadian researchers, a few body measurements and a simple calculation can produce an answer that’s accurate to within two inches for boys and 2.5 inches for girls.

“People have always wanted to know how tall their children are going to be,” said Adam Baxter-Jones, Ph.D., a kinesiologist at the University of Saskatchewan here. While several current methods exist, they are invasive, expensive, or relatively imprecise, he said.

The current gold standard, utilizing skeletal age, requires radiologic measurement of bone age to account for variation in biological maturity.

Using the data from two longitudinal studies of children in the Canadian province of Saskatchewan, Dr. Baxter-Jones and colleagues came up with a link between biological maturity and current height that can be used to predict adult height, they reported in the October issue of the Journal of Pediatrics.

“All people follow the same pattern of growth - first the legs grow and then the trunk,” Dr. Baxter-Jones said. They also go through a growth spurt - which the researchers dubbed the “peak height velocity” - during which boys grow at a rate of about four inches a year and girls at about three inches a year.

For kids between the ages of eight and 16, three measurements - standing height, sitting height, and weight - can be used to estimate how long before the growth spurt, Dr. Baxter-Jones said. Since it’s known that at the growth spurt, a child will have attained 92% of his or her adult height, it’s then possible to calculate the final height.

The method takes into account the biological age of the child, designating him or her as early-, average-, or late-maturing, depending on when the growth spurt is expected.

It’s accurate to within 2.1 inches 95% of the time in boys and to within 2.68 inches 95% of the time in girls, the researchers reported.

The researchers developed the regression equation they use by examining growth patterns in the two Saskatchewan childhood growth studies and then testing its validity in a Belgian cohort of children.

The resulting equation is simple enough that it can be automated, Dr. Baxter-Jones said, and an online version is at http://www.usask.ca/kinesiology/research_index.php.

On the other hand, the researchers noted, almost all of the children in the three studies were Caucasian. So the study will need to be replicated in other ethnic groups before it can be widely used, they said.

Another limitation pointed out by the authors is that the method should only be applied to healthy children. “We strongly suggest that the method presented in this paper only be used in children free of growth-limiting disease,” they said. “Caution should be taken when using this equation to predict heights of children with abnormal growth (i.e., individuals with hyperthyroidism or hypothyroidism, and so forth), as the reference standards and maturity predictive equation presented in this paper are modeled on a population of normal-growing children.”

The advantage of the new test, Dr. Baxter-Jones said, is that it includes a measure of biological age, while remaining noninvasive.

Previous methods have used secondary sexual characteristics as a marker of maturity, but those lack precision, as well as being invasive, he said. Predictive equations of adult height that use x-ray examination of the wrist bones to assess skeletal age are the “gold standard,” he said, but these are expensive.

The modified Roche-Wainer-Thissen method and the Khamis-Roche method, both widely used, estimate adult stature from current age, height, weight, and the average height of the parents, but they don’t include a measure of biological age, Dr. Baxter-Jones said.

One application of the finding - aside from setting parents’ minds at rest - is that it may allow people involved in youth sports to make a better assessment of the potential of children they coach, Dr. Baxter-Jones said.