One reason, is pregnancy or taking birth control pills. Moreover, Caused by estrogen and progesterone with the creation of high volume while pregnant to stimulate melanin increase. Make the skin darker and more prominent in the armpit, groin, abdomen and the nipple.(also called ceiling while pregnant). Which basically after birth will disappear or, better. Based on the texture ceiling. If the ceiling is shallow to heal, But if deeper ceiling will fade slowly but may still remain