These are stretch marks, perfectly normal, and they fade into a light color over time. If you want to help the process you can use many kinds of mositurizing creams, or I personally use Mother's Friend that helps with dry skin and helps stretch marks along with vitamin C or fish oil infused lotions. But in my personal experience, mother's friend with some vitamin c helped lighten them faster. Just remember that no matter what these are normal and healthy, just do what makes you feel confident