The linea nigra typically becomes darkened and more visible around the 23rd week of pregnancy, according to What to Expect. Latin for "black line," this line runs from the navel to the pelvic bone on women. It is always there, but when not expecting, it is lighter, called linea alba.
Hormones produced during pregnancy are responsible for the darkening of the line, according to What to Expect. Other areas of a women's body, such as the face, may also become hyperpigmented around this time. After birth, the line fades away within a couple months. To minimize excess darkening of the line, avoiding exposure to the sun is recommended, as it can intensify the discoloration.