If the black line on the abdomen that starts at the navel and goes all the way to the vulva is clinically known as the median abdominal line. This line is present in everyone, except that due to differences in skin color, some people do not show a black line. The median abdominal line runs from the navel to the vulva and is a normal physiologic phenomenon in the human body. The median abdominal line does not rise above the skin and is usually painless and itchy without any discomfort. It is present from birth and has no effect on health and does not require special treatment. Due to differences in skin color, fatness and thinness, some people show a black line on the stomach, while others have a light brown line, which can be clearly seen in some people and not so obvious in others. Usually, the color of the mid-abdominal line will deepen after pregnancy. All in all, a black line on the stomach is a normal physiological structure.