Fingernails are made from keratin, a protein and grow from a root (matrix) which is affected by the state of health of the rest of the body. If nails become misshapen, ridged or discoloured, it can be indicative of an underlying illness, so it’s worth getting things checked out if you notice that your fingernails are doing odd things. A few years ago, the nails on my little fingers became very soft and began to take on a slight upward curve. What I didn’t know at the time is that this is referred to as "spoon-shaped" nails (though they didn't really look that spoon-like to me), and are often associated with anemia. In severe cases of anemia, the nail will continue to change shape until it can actually hold as much as a drop of water in the curvature. I didn't experience anything that extreme because I went to a doctor, held my hands out, said, "What's this about?" and he made me take huge doses of iron.

Aside from having your fingernails turn into little spoons, there are some other changes that one might want to ask a doctor about, such as:

When the fingertips widen, become rounded and the nails curve around them, this is known as "clubbing". It's due to a drastic enlargement in connective tissue as it tries to compensate for a chronic lack of oxygen and so is often associated with people who have lung disease, heart disease, or cancer.

An arc of brownish discoloration, "Half-and-Half (Lindsay's nails)" appears in a small percentage of people who are experiencing kidney failure.

White lines that run horizontally across the nail, following the shape of it's "moon" are known as "Mee's lines," often attributed to arsenic poisoning.

Indentations that run horizontally across the nail are known as "Beau's lines" and can appear as a result of growth being stunted at the root (matrix) due to severe illnesses such as a heart attack, measles, pneumonia or high fever.

There is a condition known as "Onycholysis" in which the nail separates from the nail bed, possibly due to physical injury, psoriasis, drug reactions, fungal disease or contact dermatitis from using nail hardeners. It may also be related to an over or under-active thyroid, anemia or syphilis.

Small pits or depressions are called "pitting," often occurring with people who have psoriasis.

If the nail looks opaque and white, but the nail tip has a dark pink to brown band, this is known as "Terry's nails". and may be indicative of cirrhosis, congestive heart failure, a warning sign of diabetes in adults, cancer or simply aging.

Narrow ridges that runs the length of the nail are simply called "Vertical ridges." They may appear in the nails of adults, become more pronounced with aging and are sometimes associated with kidney failure.

When one or more nails turn yellow or green, this is called "yellow nail syndrome." The nails grow slowly and the cuticle disappears, sometimes due to fluid retention in the hands and feet or a number of respiratory diseases, such as chronic bronchitis.

Source: My own fingernails and information from
(originally published in a Mayo Clinic Health Letter)

And by the way, it is estimated that one in four women are anemic and don't know it.