Fingernails grow slowly — roughly 3mm a month, so a full nail takes about six months to replace itself.
That single number explains most of what people find alarming. A ridge, a groove or a white line appearing now was made weeks ago, and the illness or injury that caused it has usually already resolved.
It also means a damaged nail bed doesn’t declare its final result for half a year.
After a crush or a nail bed injury
The nail plate is just a hard covering. The thing that decides how the nail grows back is the bed underneath it and the matrix at the base, hidden under the cuticle.
Damage the matrix and the nail grows back split, ridged, thickened or not at all. Damage only the bed and it usually recovers well. This is the reason a crushed fingertip is about the nail bed rather than the fracture.
- Week 0 to 4 — the old nail often lifts and falls off. Normal. Don’t pull it early; it’s a dressing
- Month 1 to 3 — new nail emerges, often thickened, ridged or discoloured
- Month 3 to 6 — appearance improves considerably
- Month 6 to 12 — final result. Nothing about a nail should be judged before six months.
Nails are a slow-motion record. What you’re looking at today happened a month or two ago.
What the common changes mean
A horizontal groove across one nail (Beau’s line). The nail stopped growing briefly. Measure back from the cuticle — roughly a month per 3mm — and you’ll usually land on an illness, a fever, chemotherapy or the injury itself.
Vertical ridges. Almost always just age. Extremely common and rarely significant.
A vertical groove running the length of one nail, on a finger with a lump at the end joint — that’s pressure on the matrix from a mucous cyst. Treat the cyst and the groove grows out.
Pitting, like a thimble. Points at psoriasis or psoriatic arthritis, particularly with sore end joints.
Spoon-shaped nails. Can indicate iron deficiency. Worth a blood test.
Clubbing — nails curving over increasingly bulbous fingertips, developing over months. That one is a genuine medical flag and needs a doctor.
The one that must not be ignored
A dark streak running lengthways down a single nail, which is new, widening, darker at the base, or spreading pigment onto the surrounding skin.
That last feature — pigment on the skin at the nail fold — is the one that matters most. It can indicate a melanoma under the nail, which is routinely mistaken for an old bruise and therefore diagnosed late.
A bruise grows out. A streak that stays in the same place, or widens, does not. If you cannot remember an injury and it has been there for months, get it looked at rather than watched.
The reassuring part: multiple dark streaks across several nails are common and usually normal, especially in people with darker skin. It’s the single, changing, widening one that warrants a look.
A bruise under the nail
Blood trapped under the nail after a crush is intensely painful because it has nowhere to go. Draining it relieves that immediately and is a quick procedure — but it should be done by someone who has first considered whether there’s a fracture or a nail bed tear underneath.
Large collections under the nail often indicate a significant nail bed laceration, which may need repairing.
Get it looked at if
- A single dark streak is new, widening, or pigment is spreading onto the skin
- One nail is thickening, lifting or crumbling with no injury behind it
- The nail fold is red, swollen and tender — that’s a paronychia
- Fingertips are becoming bulbous with curving nails
- A crushed nail is under tense pressure from trapped blood
Most nail changes are history, not news. The exception is a single dark line that is changing — and that one is worth a photograph and an appointment.
Every article on this site is written using the Closing Window Method: name the window your condition runs on, find where you are in it, and act while it’s still open.