How do your legs and arms know to grow at the same rate?

Bilateral symmetry is generally a sign of health and reproductive success. Large deviations from symmetry are maladaptive.

Growth hormones affect the entire body, and so regulate growth in general rather than targeting specific limbs.

They have the golden ratio as a guideline.

There is no simple answer to your question as this remains an active area of research. Embryology in general and limb development in particular teaches that specific factors such as fibroblast growth factor (FGF) in concert with the timely activation of homeobox (hox) genes leads to the ordered development of limbs. Specifically, hox genes are transcription factors (DNA binding proteins) that coordinate the expression of a whole set of genes in a given pattern, leading to the appropriate development of specific organs and body parts. Eventually, once the initial framework is built, then hormones including growth hormone, testosterone and estrogen affect gene transcription as growth occurs before and during puberty. Once the growth plates of your bones are fused, longitudinal growth ceases.

Thanks, Doc!

Because…

This little piggie went to market,
This little piggie stayed home.
This little piggie had roast beef,
This little piggie had none.
And this little piggie cried “wee, wee, wee,”
All the way home.

My arms have yet to grow :(

One of my legs is marginally shorter than my other leg… * cries *

I…*sniff *... am not symmetrical!

Wowwwwww that’s so interesting!!! The human body is a FREAK THANG.

Even more amazing?! The same process that occurs in humans occurs in the simple fly which is a terrific example of evolution (i.e. an advantageous process is maintained over time).

My hair grows at the same rate.

Forget about legs and arms. I am worried about arm pits.