Still, medical evaluation is particularly important when:
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A new dark streak appears without an obvious injury.
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The pigmentation becomes wider or changes over time.
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The color is irregular rather than uniform.
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Dark pigment appears on the skin surrounding the nail.
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The nail becomes progressively distorted or damaged.
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The discoloration remains in place instead of growing outward with the nail.
These signs don't mean you have cancer. They simply mean that the cause should be identified rather than assumed.
Your Nail Can Give You a Timeline
One useful thing about fingernails is that they grow slowly.
If a dark area was caused by an old injury, new nail typically grows from the base and gradually pushes the affected portion toward the tip.
Over time, you may therefore see the discoloration move outward.
This can help distinguish an old traumatic injury from some other causes of persistent pigmentation.
Taking a photograph every few weeks can also help you notice whether a nail is actually changing. If the appearance continues to worsen or you aren't sure what you're seeing, a dermatologist or other healthcare professional can examine it properly.
When Should You Get It Checked?
Don't panic if you notice a dark thumbnail. Instead, consider what happened before the change appeared.
If you recently injured the finger and the discoloration developed afterward, trauma may be a reasonable explanation.
But it's a good idea to seek medical advice if: