1. Why Nails Look Pink and What the Color Shows
Look at your fingernails under a bright light. Most of the time, they look light pink all over. This is because the tiny blood vessels under the nail shine through the hard surface [3]. Think of your nail as a small window that lets you see a little bit of what is happening inside your body. Because of this, people have long connected changes in nail color or shape to health conditions.
However, you should not worry too quickly. Many changes in nail color are caused by problems with the nail itself, such as a fungal infection. Other causes can include kidney or liver issues, certain medicines, or just an injury or irritation [1]. It is best to view these signs as possible messages from your body, not definite diagnoses. So, if you see a line in your nail, how do you know if it comes from your body or just the nail? One important clue is whether the line moves or stays still.
2. How to Tell if a Nail Line Moves or Stays Put
Imagine a sticker on a moving walkway at an airport. The sticker moves with the walkway. Now imagine a sticker on the floor next to it. Even when the walkway moves, the floor sticker stays in the same spot. Nails work in a similar way. A groove that runs across the nail is called a Beau line. These lines can happen after an infection, an injury, a serious illness, or chemotherapy. They occur because nail growth was briefly stopped [1]. Since the line is part of the nail, it moves forward from the root toward the tip as the nail grows.
You can use this movement to guess when the event happened. Fingernails grow about 3 mm (about 0.12 inches) per month, while toenails grow about 1 mm (about 0.04 inches) per month [4]. For example, if a line is 6 mm (about 0.24 inches) from the root of your fingernail, it likely happened about 2 months ago. If it is on a toenail, the same distance means it happened about 6 months ago, because toenails grow slower. These are rough estimates, as growth speed varies from person to person.
On the other hand, Muehrcke lines are pairs of white bands that go across the whole width of the nail. These are linked to low levels of a protein called albumin in the blood. Unlike Beau lines, Muehrcke lines do not move as the nail grows. They also disappear when you press on the nail. This is because the change is in the nail bed underneath, not in the nail itself [5]. So, moving lines often point to something that happened while the nail was being made, while still lines point to changes under the nail. Note that not all lines fit this rule perfectly; for example, Mees lines do move with the nail. Also, Beau lines are horizontal grooves caused by a disturbance in nail formation, often after illness or injury [1].
3. Spoon-Shaped Nails: Not Always a Sign of Low Iron
Sometimes, the center of a nail dips down and the edges curve up, making it look like a spoon. This is called a spoon nail, or koilonychia. It can happen with iron deficiency [1]. However, you cannot assume it is always caused by anemia. Other causes include pressure on the fingertips or cutting the nails too short at the corners. In babies, spoon nails can be a normal variation that often improves on its own as they grow up [5].
Medical books also list other conditions that might cause spoon nails, such as hemochromatosis (where the body stores too much iron), trauma, or Raynaud’s disease (where blood flow to the fingers is reduced by cold) [5]. This means that seeing a spoon shape alone does not tell you the exact cause. Another well-known sign is clubbing, where the fingertips become swollen. This condition has been recorded for over 2,000 years.
4. What White Nails (Terry Nails) and Black Lines Mean
Terry nails are mostly white, looking like frosted glass, with only 1 to 2 mm (about 0.04 to 0.08 inches) of normal pink color left at the tip. The name comes from Dr. Richard Terry, who in 1954 noted that 82 out of 100 patients with liver cirrhosis had white nails [6]. This was just the first report. It does not mean everyone with liver cirrhosis gets white nails. Later, these nails were also seen in people with heart failure, type 2 diabetes, chronic kidney failure, and tuberculosis. They can also appear just because of aging [4]. However, if a young person has Terry nails with no known cause, doctors may pay closer attention.
Another sign that worries people is a vertical line that is gray or black. These lines run from the base of the nail to the tip. They can appear naturally in people with darker skin tones. But because they can rarely be an early sign of cancer, you should talk to a doctor if you notice a vertical line that concerns you [1]. On the other hand, white horizontal lines or split nails are often caused by everyday things like dryness or bumps to the nail. Causes vary, and you cannot judge health just by looking at nails [3].
5. Clubbed Fingers: A Nail Sign Noticed for Centuries
Clubbing is when the fingertips become round and swollen, like the head of a drum, and the angle at the base of the nail changes. It is not painful [2]. Medical history says that around 400 BC, the ancient Greek doctor Hippocrates first described this in patients with empyema (a chest condition). Because of this, it is sometimes called Hippocratic fingers. This story comes from medical texts, though we have not checked the original ancient writing here [7].
Clubbing can be linked to lung diseases like lung cancer, interstitial lung disease, or bronchiectasis. It can also be associated with congenital heart defects or liver disease. Interestingly, pneumonia and asthma are not listed as common causes. Sometimes it is genetic, or the cause is never found [2]. Also, even if clubbing is progressing, the person often does not notice it themselves [7]. This is why it is valuable when someone points it out or it is found during a health check-up.
6. Why Nail Signs Cannot Diagnose a Disease Alone
The signs we have looked at—Beau lines, spoon nails, white nails, and clubbing—cannot diagnose a disease on their own. Beau lines are marks of temporary weakness. Spoon nails can sometimes be caused by things like pressure on the fingertips. White nails and clubbing have many possible causes. Most changes in nail color or shape are due to nail problems themselves, like fungal infections [1]. White lines and split nails can also come from dryness or minor impacts. Because causes are varied, you cannot judge your health just by looking at your nails. If symptoms continue, seeing a dermatologist is recommended [3].
Think of nail signs as an "entry point," not a final answer. They can be a clue, but the decision belongs to a doctor who looks at your whole body. Do not judge signs by yourself. If a sign continues or gets worse, tell an adult and see a doctor.
7. How to Measure Your Own Nail Growth with a Ruler
You can try a simple experiment today. On the day you cut your nails, use a ruler to measure the length from the skin at the base of the nail to the tip. Do not cut your nails for two weeks. Then, measure the same finger in the same way. Fingernails grow about 3 mm (about 0.12 inches) per month on average. So, in two weeks, you might expect about 1.5 mm (about 0.06 inches) of growth. Compare your result. Everyone is different, so it is okay if the number does not match exactly.
If you want to read more, the "MSD Manual Home Edition" page on nail deformities, dystrophy, and discoloration is available in Japanese. You can look at photos of signs you are curious about with an adult [1].