A line in the middle of a pregnant woman’s abdomen is the stretch mark, which is a normal physiological reaction during pregnancy. The essence of the stretch mark is caused by melanin deposition in the area of the abdominal white line, which is a normal anatomical structure of the human abdominal wall and exists in all people and is only inconspicuous because of the lighter pigmentation. Due to the increase in hormone levels in pregnant women, including estrogen and progesterone levels, the metabolism of melanocytes becomes abnormal, resulting in excessive deposition of melanocytes and deepening of local pigmentation and the formation of stretch marks. Most pregnant women have stretch marks on their abdomen, but the color of stretch marks varies from individual to individual, and may be related to the skin tone and texture of the pregnant woman. Pregnant women with fair skin and who are not prone to acne marks may have lighter stretch marks, while pregnant women with darker skin and who are prone to pigmentation, such as wounds that leave marks, may have darker stretch marks. Most of the colors will gradually fade as the hormone levels in the body return to normal after delivery.