Look closely at your nails and you may one day spot a groove running across them, from side to side. Patients would sometimes point one out to me with real worry. In most cases, the answer was reassuring, and rather fascinating: the nail had simply kept a record of something that happened weeks or months earlier.
These grooves have a name: Beau’s lines, after the French physician Joseph Honoré Simon Beau, who first described them in 1846.
What Causes Them
Every nail grows from the matrix, a band of living tissue hidden under the skin at the base of the nail. Normally, it produces nail at a steady rate. If something interrupts that production for a while, the nail made during that time is thinner. As it grows out, the thin section shows as a groove running across the nail.
Anything that briefly slows the matrix can do it. Common causes include:
- An injury to the nail, such as stubbing a toe hard, dropping something heavy on it, or trapping a finger in a door. Tight footwear can do it too.
- A significant illness, especially one with a high fever.
- Some medical treatments, notably chemotherapy, which targets fast-dividing cells, including those in the nail matrix.
- Poor nutrition or other conditions that affect the whole body.
One Nail or All of Them?
This is the first clue I would look at. A line on just one nail usually points to something that happened to that nail, most often a knock or pressure. Lines on several nails at the same level suggest something that affected the whole body at once, such as an illness.
Working Out When It Happened
Because nails grow slowly and fairly steadily, a Beau’s line works like a date stamp. It starts at the base of the nail and moves towards the tip as the nail grows. A line from an illness usually becomes visible about 2 to 3 weeks afterwards, once the affected nail has grown out from under the skin.
To estimate when it formed, measure from the cuticle to the line. As a rough guide, fingernails grow about 3.5 mm a month and toenails about 1.6 mm a month:
| Distance from cuticle | Fingernail: time since it emerged | Toenail: time since it emerged |
|---|---|---|
| 2 mm | 2–3 weeks ago | 5–6 weeks ago |
| 5 mm | 6 weeks ago | 3 months ago |
| 10 mm | 3 months ago | 6 months ago |
Then add the 2 to 3 weeks the line took to appear. So a line 5 mm up a fingernail points to something that happened roughly two months ago. These are only estimates. Nails grow at different speeds in different people, and more slowly with age. But it is often enough to connect the line to a bout of flu, an operation or a stubbed toe.
When to Get It Checked
A single line after a known injury or illness is nothing to worry about. It is worth seeing your GP if lines appear on several nails with no obvious cause, if they keep coming back, or if your nails show other changes, such as colour changes, thickening or the nail lifting away. Occasionally, nail changes are the first visible sign of a condition that needs attention.
The Bottom Line
Beau’s lines are a record, written in keratin, of a time when your nails briefly paused. Most tell a simple story of a knock or a bout of illness. Read them well, and your nails can tell you roughly when it happened.
Sources
- Beau’s lines and onychomadesis: a systematic review — Acta Dermato-Venereologica, 2023
- Beau lines associated with COVID-19 — Canadian Medical Association Journal, 2020
- Average fingernail and toenail growth rate — BioNumbers (Harvard), citing Yaemsiri et al., 2010