Definition: Stretch marks occur when the dermis, the middle layer of your skin, is stretched to a point where its elasticity begins to break down. At this point, microscopic bleeding occurs along with tissue inflammation which results in raised, reddish purple lines, or striae, of newly formed stretch marks. These will eventually fade to a silvery white, a few shades lighter than your natural skin tone. 
Skin does not bounce back if it’s been stretched by rapid growth due to pregnancy, weight gain, or extreme weight loss.  These are the most common causes of stretch marks. It is estimated that up to 98% of women and 75% of men have stretch marks. These typically occur on the hips, breasts, thighs, and abdomen of women, and the buttocks and pectoral area of men. 
As the dermis is stretched, the outer layer of skin, the epidermis, also stretches making it translucent enough that you can see the reddish purple stretch mark forming. As these marks heal, scar formation occurs, and the marks fade to a silvery white color that is a few shades lighter than your natural skin tone.  The lighter color occurs because during the overstretching of the dermis, natural collagen production gets disrupted.  This in turn may cause loss of skin pigment producing cells  (melanocytes).
There are several treatment options for stretch marks. The degree of success with any treatment will depend on age, skin tone and overall well-being including exercise and diet.
A recent study performed by Dr. Zoe Draelos, a consulting professor of dermatology at Duke University Medical Center in Durham, N.C, in association with Merz revealed that a moisturizing cream whose active ingredient is extract of onion can help take the redness out of new stretch marks.  The new study involved 54 women, aged 18 to 45, with new, red stretch marks on their outer thighs. They worked a quarter-sized amount of the cream into one of their stretch marks twice a day for 12 weeks, and another was used for control with no treatment performed. 
As judged by the women who were subjects in the study, the treated stretch mark looked better, was less red, and was softer and smoother than the untreated stretch mark. After two weeks of treatment results were seen, and the difference persisted through all 12 weeks of the study.  The researchers also noted that the treated stretch mark looked and felt better beginning with the second week, compared with the stretch mark that was not treated. But it wasn’t until the eighth week of treatment that they noticed a substantial difference in redness.  None of the women reported any side effects, and the treatment was not tested in men. 
Why Onion Cream Works
Any damage to the skin, including 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.  The new product developed by Merz also contains pennywort, a plant found in Asia, Africa, and the Americas that has anti-inflammatory properties and is widely used in Indian naturopathic medicine for ulcer healing.  The cream also includes sulfur to combat infection and a moisturizing cream to rehydrate the skin. 
The study, funded by Merz Pharmaceuticals, which makes the cream, was presented at the annual meeting of the American Academy of Dermatology. 
Called Mederma Stretch Marks Therapy, a 5.29-ounce tube retails for $39.99 at drugstores, according to a Merz spokesperson. 
Caution to Consumers
The design of this study was such that the onion cream treatment was almost guaranteed to come out ahead because this cream was compared to an untreated stretch mark. There are several other moisturizing creams that claim to reduce stretch marks. Additionally, these study participants were un-blinded, meaning they knew what stretch marks were being treated. Therefore, the patients could perceive benefits from this treatment regardless of whether the therapy was objectively effective (the placebo effect).
Dermabrasion, chemical peels, or laser surgery can be used to address unwanted stretch marks. For laser surgery, usually the same lasers used to treat vascular problems, which are yellow- and green-light lasers, are used to treat stretch marks. This likely has to do with the ability of collagen or elastin molecules to absorb the light. It may only require repeated treatments and improvement may occur slowly and take up to a year for all of the improvement to take place. 
Approximately half of patients who undergo laser treatment for stretch marks feel that there is an improvement.  True testing of improvement is difficult to discern because stretch marks do not photograph well and can seem less apparent from one angle than another. In addition, younger patients seem to have the best outcomes, as do those that have stretch marks that are thin and pink. Flat, white stretch marks do not appear to have as much improvement following laser surgery. 
Other Lotions and creams
Other over-the-counter stretch mark treatments are available. If you are pregnant, discuss your concern with your doctor at the beginning of your pregnancy, before stretch marks develop, so that preventive treatment can be started. 
Diet and exercise
Adequate hydration keeps your skin soft and less likely to develop stretch marks, so drinking water is important. Caffeine can increase your risk of stretch marks. Drink just as much (or more) water as you drink coffee, tea, or soda.  Stretch marks can also result from nutritional deficiency. Be sure to consume foods that promote skin health: foods rich in zinc, such as nuts or fish; foods high in vitamins A and C, such as carrots and citrus fruits and milk; protein-rich foods, such as eggs. 
Does insurance cover surgery to remove stretch marks?
Insurance may not cover stretch mark removal because it is a cosmetic procedure (even if the stretch marks are severe).  Talk to your particular insurance provider to see what is covered under your plan.