The dark line that leads from the belly button to the pubic area is called linea nigra. This line appears in the second trimester of pregnancy and is associated with estrogen levels and melanocyte stimulating hormone deposits under the skin. Women who are dark-skinned tend to have a darker linea nigra than women who are fair-skinned.
After pregnancy, linea nigra may fade but remain visible for some women. During summer months when midlines may come in contact with sunlight, the line may darken the line so it is important to continue wearing sunscreen in order to keep the darkness of the line to a minimum.