How tall will I become?

Alternatively: how tall will my child become?

As usual, I'll use the question to tell you a bit about the science of growth... and then answer your question. Hope you don't mind!

There are four phases of human growth.
Fetal - By the time you are born, you have usually already obtained 30% of your eventual height! The chief determinants here are the size of the mother and the ability of the placenta to provide adequate nutrition.

Infantile - This covers the period from birth until 18 months, and contributes around 15% of your final height. Growth in this phase is largely dependent on adequate nutrition.

Childhood - The period from 18 months to puberty accounts for about 40% of your eventual height. Assuming adequate nutrition, the main driving force behind the growth is a hormone, appropriately named growth hormone, backed up by other hormones, including the thyroid hormones and steroids.

Pubertal - The sex hormones (chiefly testosterone in a male, and oestradiol in a female) boost growth hormone secretion and add the final 15% onto your height. After this, we stop growing. This happens at about 18 in females and 21 in males, though this number varies.

Obviously, growth in any of these stages can be limited by pathologies, including poor nutrition, chronic diseases, broken bones, thyroid problems and even psychological problems. Overall, in a first world country, the factors accounting for the difference between your height and the next persons are about 80% genetic and 20% environmental.

Why do we stop growing? Our height is basically determined by the lengths of our bones, chiefly the 'long bones', like the femur (thigh bone) and humerus. They grow by means of a growth plate, which is a cartilaginous area of the bone where the cells gradually develop into bone. This new bone on either side of the growth plate slowly moves further apart as more new bone is created... and thus the bone lengthens. Growth stops when the growth plate fuses, since no new bone can be created.

Why are men taller than women? Actually they only end up taller (and obviously only on average - many women are taller than many men). The difference only really becomes apparent in the pubertal stage. Girls usually hit it earlier than boys do - at around 11 to 13 years, which is why they often become temporarily taller than the boys then. However, their growth spurt is fuelled by oestrogen, which tends to cause early fusion of the growth plates. Boys, on the other hand, are propelled upwards in this phase by testosterone. This usually happens somewhere between 13 and 17. However, testosterone takes much longer to fuse the growth plates, and so men end up taller.

(Couldn't the body have been 'designed' so that oestrogen didn't fuse the growth plates earlier than testosterone? Yes of course it could have. Thus, you are quite entitled to rephrase the (deeper) question as, "Why are men 'designed' to be taller than women?". The answer to this beyond the scope of this topic, but here's a clue: it's for the same sort of reason than Elephant Seal males are four times as big as the females. If you're interested, send an email or leave a comment to that effect...)

And so finally... how tall will I become? No one can obviously say for sure, but there are several formulas that provide a reasonably accurate guess.

The one I like goes like this:

• (mother's height + father's height) divided by 2
• then, take this number and add about 6.5 cm (2.5 inches) to it for a male, or subtract this amount for a female. You can have this worked out for you at this website here.

This formula largely considers only the genetic component to growth. A more complicated formula, taking into account some of the environmental aspects too, can be found here.

Another rough rule of thumb is to take the child's height at 3 and double it. (Yes, you are roughly half you total height at the measly age of 3!) However, between a male and a female of the same height at this age, the male will usually end up the taller, as alluded to above. A more complicated formula which factors this in is as follows:

• For a male: (Height-at-3 + 22 inches) x 1.27
• For a female: (Height-at-3 + 17 inches) x 1.29

[22 inches ≈ 65 centimetres; 17 inches ≈ 43 centimetres]

There are numerous other formulas and methods available, but the above are quite enough! Remember, again, that they are only a guide.