Stretch marks are fairly common and happen under the epidermis, in the dermal layer of skin - where the collagen lives. During a fast growth phase, pregnancy, or if weight is gained quickly, the collagen ‘weave’ can be torn. It is stretched further than it is able to recover and this then looks like a light or dark wrinkled line, under the epidermis, which remains intact.
The recovery of these areas depends on skin type, how long they have been there, how many have formed, diet, age (and collagen condition) and melanin pigmentation.
You can simply try rubbing in an emollient oil: coconut, argan, rosehip, marula, wheatgerm or rice bran oils. In some people the combination of the properties in the oil and the frequent massaging of the area improves the marks enough for them not to be a concern. It is worth trying this first, before spending money on more complex mixes.
If you need something more, vitamin A and C are likely to help, both in your diet and applied topically. They tend not to be formulated together as, depending on the types uses, they work better in a different pH. Vitamin C at about 8% mixed with 2% vitamin E and .5% ferulic acid will help smooth the top layer of skin, fade the pigmentation, and encourage collagen production. Vitamin A (there are several forms) works well with hyaluronic acid, allantoin, niacinamide and panthenol to improve the quality of the epidermal layer and can also increase elastin and collagen production.
If you are unable to find these ingredients, botanical extracts can work too, and you can make these at home. Borage, rose, honeysuckle and calendula are all good. You can either soak the flowers in grapeseed or olive oils, with 10% vitamin E for 3–6 weeks and then apply it to the affected area. Or you can also soak them in glycerine and distilled water (60/40) for the same time. Make sure the jar is air-tight and you have filled it to the top (if any flowers are sitting above the liquid, these will deteriorate fast and could ruin the whole batch). For either, use roughly 1/3 weight in flowers to liquid. You can also use dried flowers if fresh ones are not available, although I don’t believe this will be as effective. Evening primrose and St John’s wort can also help, but ensure you don’t go into the sun after applying St John’s wort as it can be a photo-sensitiser and can cause an allergic reaction in some people, so test it on your inner arm first. Evening primrose shouldn’t be used if you suffer from epilepsy. Alternatively, you can distill these flowers to produce a hydrosol, which can also be beneficial. One of the best for repairing skin is a seaweed - Bladder Wrack -fucus vesiculosus, you can get this as an extract or a hydrosol.
Diet is alway important; ensure you are taking sufficient nutrients for your skin to repair. There have been some interesting trials on ingesting collagen to improve the amount that makes it into skin. It seems promising but not conclusive, and fish appears to be one of the best sources. If you are vegetarian, soy and soy products have done well, nuts and seeds may also help. You can buy collagen supplements, but the regulations on these are not stringent, so establishing what, exactly, and how much you might be swallowing can be difficult.
It will take a while to see a marked improvement and you may have to try a few combinations to find which is most effective for your skin. It is worth persevering, give it at least three weeks to see any improvement.