Simply put, hyperpigmentation is the darkening of the skin or the nails due to excessive production of melanin in the body. Most pregnant women experience hyperpigmentation in some or other form.
Sometimes it can manifest itself by way of the mask of pregnancy; also known as Chloasma or Melasma. This is usually a butterfly shaped patch on the face and it may also manifest itself on other parts of the body such as the arms. Chloasma can get worse with successive pregnancies. 
Many women experience hyper pigmentation in the form of the linea nigra that forms in the middle of the abdomen during pregnancy. The linea nigra originates at the navel and extends down to the pubic bone, seems to bisect the abdomen into two perfect halves.
The pigmentation of darker areas of the body will become more concentrated during this time. So, if a woman has freckles or moles, she will find that these markings will become darker and more prominent at this time.
Even those women who do not experience noticeable conditions like Chloasma will most often notice a darkening of the nipples and aureoles as well as the genital area during pregnancy.
Most forms of hyperpigmentation fade and disappear soon after childbirth and are nothing to worry about.
