Answer: Hair Whorls

If your hair isn’t particularly unruly, you may have never given much thought to the whorls, or “cowlicks” of your hair (while those of us with pronounced hair whorls spend quite a bit of time subduing them to avoid looking like mad scientists). They seem like a benign curiosity of human development and little more, yet doctors observing a newborn baby find them particularly useful in assessing brain development.

How can the pattern of a newborn’s hair tell a doctor anything about their brain development? It isn’t that brain development directly influences hair growth or that hair growth directly influences brain development, but that the growth of the scalp skin and hair follicles occurs in parallel with the growth of the brain in utero, and if the hair development is abnormal, it tells the doctor to investigate further.

The same “if this is abnormal then that might be too” rule is used for other diagnoses too: the ears develop in parallel with the kidneys, so if a newborn has deformed ears, doctors will run tests to ensure they have healthy kidneys.