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At the base of fingernail, there is a white semicircle (see on the first picture). But sometimes there isn't one.

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My questions are: what is this white semicircle on the fingernail, and what causes it? What is the explanation when there it isn't on all of the fingernails (see on the second picture)?

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The lighter crescent-shaped part at the base of the nail is called the lunula. It is a normal part of the nail matrix, and indicates nail growth. The faster your fingernails grow, the more obvious the lunula.

enter image description here

Absence of the lunula without any other abnormalities of the nail is not uncommon, and while it can indicate anemia or malnutrition, it is often found normally, and with aging. In this case, careful inspection of all the nails will usually reveal a lunula on the thumbs and the fingers closest to the thumb, but a very small or absent lunula on the other fingers.

In your picture, there is clearly a lunula on the middle finger, and, I suspect, a larger one on the index finger, and a relatively normal one on the thumb. This is not a finding of great concern in the absence of other pathology.

The lunula: The small, whitish, half-moon shape that you sometime see at the bottom of your nails is called the lunula(pronounced loon-yoo-la). It's actually part of the matrix. You might be able to see it only on your thumbs, or maybe not at all. Don't worry if you can't see a lunula on any of your fingers. It's no big deal. It's there, just under your skin.

Examining the Fingernails When Evaluating Presenting Symptoms in Elderly Patients
Examining the Fingernails
Nails

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protected by Chris May 3 '16 at 5:08

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