Puberty, pregnancy and rapid weight gain are three sources of stretch marks – those permanent lines that form (hips, breasts, thighs and stomach) when skin is stretched rapidly. Moisturizing creams with an active ingredient called onion extract holds promise to reduce the redness of new stretch marks. They’re not harmful to your health, but many people are bothered by their appearance. The marks slowly fade and become flat on their own, but that can take years. Any insult to the skin — be it a cut or wound or the rapid stretching that drives the formation of stretch marks — is accompanied by inflammation. Onions contain flavonoids, a type of antioxidant, that have anti-inflammatory properties.