During pregnancy, an influx of hormones stimulate melanocytes, which are specialized skin cells responsible for the production of melanin. Melanin occurs in all human beings (with the exception of those with albinism) and is responsible for the pigmentation of skin. Tanning from UV exposure is the most common reason for an increase in melanin production, but pregnancy-related hormonal changes can also cause these changes.
A pregnant woman's body increases melanocyte production and activity in order to darken her areola to make it an easier, more visible target for the baby. As a side effect of this phenomenon, some women also develop pigmented patches of skin, or a darkening of the face known as chloasma. One of the clearest and most common effects of the phenomenon is the linea nigra.
The appearance of the linea nigra varies between individuals. Very light-skinned women are somewhat less likely to develop the line. In women with skin of medium tone, the line will generally appear chocolate brown. Olive-skinned women like me might develop a coffee-colored linea nigra, while women with darker skin may have a black line. The line may start as a linea alba -- a light-colored or white line -- before it develops deeper pigmentation.
Some experts, including the staff behind "What to Expect" and BabyZone, hypothesize that folic acid supplements may help to reduce the appearance of the linea nigra. This claim appears to be nothing more than a modern-day old wives' tale. No peer-reviewed studies published on PubMed reveal any evidence to this effect. However, folic acid supplements are widely recommended during pregnancy and included in most prenatal multivitamins, so it can't hurt to use them in hopes that they might reduce the appearance of the linea nigra.
Ultimately, though, there is no reason to be concerned or self-conscious about the appearance of the linea nigra. I believe that it is one of the most beautiful features of an expectant mother's body, and it is not a sign of any kind of health problem, deficiency, or complication. If you have any concerns about your linea nigra or its appearance during pregnancy, talk to your prenatal health care provider for more information about the phenomenon and its causes.