Your post pregnancy belly develops stretch marks and linea nigra (a dark line in the middle of the abdomen) as a result of excessive stretching of the skin during pregnancy. (newkidscenter.com)
Shortly, you may notice a dark vertical line of skin pigmentation running down the middle of your abdomen, from your belly button to about where your pubic hair begins (the center of your pubic bone). (pregnancy-baby.com)
A dark line called the linea nigra that runs from the middle of the abdomen to the pubic area may appear. (doctor-info.website)