After pregnancy, the belly button becomes black due to the increase of melanin secretion in the body. After pregnancy, not only the belly button will become black, but also the neck, armpit area, thigh root area and vulva area will become black, which is a normal physiological phenomenon. After delivery, it will gradually return to normal within six months, do not worry, after pregnancy, due to the increased burden on the maternal tissues and organs, the metabolism increases, must pay attention to supplemental nutrition, pay attention to eat more high-protein food, drink more pure milk, yogurt, maternal milk powder, eat more fresh vegetables and fruits, fish, shrimp, lean meat, etc., should also pay attention to eat more. Pay attention to go to the hospital on time for maternity checkups, to be able to rule out fetal development problems during pregnancy, and if there are really problems, they can be found in time. During pregnancy, it is important to properly abstain from sex.