The linea negra, which literally means "dark line" is something that most pregnant women get. It happens to women with all skin colors, but it's likely to be darker in those with darker skin. This "war stripe" is caused by the same thing that causes other skin changes in expectant mothers you can blame your hormones for this one too! But unlike impurities of the facial skin, or pregnancy acne, this skin change can actually be very beautiful. This dark line can show up at any time during pregnancy or not at all. Don't worry if you do not get a dark line on your belly. Although most women notice some skin darkening around their belly, it is not a cause for concerns if you do not notice any yourself.
Most women who get a linea negra notice it sometime during the second trimester, but others notice it practically as soon as they take their pregnancy test, while some do not get it until they are about to give birth. Funnily enough, there are some ladies who will first see that their belly button starts getting darker, and a line will develop only after that. A linea negra is not permanent. It will start fading in the months after your baby's birth, and eventually it will just disappear.