Nails that curve downward, a condition known as nail clubbing, indicate that there could be an underlying health condition. Most often, these issues involve diseases of the lung and cardiovascular system. Nail clubbing may also indicate that the level of oxygen in the blood is low, according to Mayo Clinic.
Nail clubbing occurs in stages. First, the nail bed becomes soft. Then, the nail starts to gently curve under. As the condition progresses, the nail becomes thicker, especially at the end of the finger. Additionally, the nail may appear to have stripes or grooves. Since many nail diseases and disorders are caused by medical conditions, it's vital to seek medical attention for any nail changes.