If you Google stretch marks in pregnancy, the first thing you will find is an advertisement for a belly sheet mask. If you Google sheet masks for pregnancy, you will get over 10 million hits. Belly sheet masks for pregnancy are like sheet masks for your face, except you use them on your belly to prevent or reduce stretch marks. 
The belly sheet mask claims to reduce or prevent stretch marks by moisturizing your belly skin. You take out a moisturized film from the package and place it over your belly, where it stays to give your skin continuing exposure to moisturizing lotions while you go about your day. It is recommended to use the sheet mask twice each week. 
Striae Gravidarum: Pregnancy Stretch Marks
Striae gravidarum are scars that appear during the second and third trimesters of pregnancy as your belly enlarges. The growth of your belly happens too quickly for the elastic structures that support your skin to adapt. These support tissues are proteins called elastin and collagen. Rapid stretching of the skin causes them to rupture. The result is stretch marks. At first, they look swollen and have a pink to purple color. Over time they may become permanent. As the scars heal, they become sunken marks with a white, shiny, crinkly appearance. [2,3]
Will a Belly Mask Help?
Since up to 90 percent of women will develop some stretch marks during pregnancy,  it is not surprising that they are lining up to try the belly sheet mask. A sheet mask works well as a facial moisturizer, so why not use one on your belly? The problem is that moisturizing alone will probably not help. The active ingredients in a belly sheet mask are aloe vera and propolis, an antioxidant made from bee’s wax. 
There have been no studies done to support the use of belly masks, but research on a variety of creams, lotions, or gels used to prevent or reduce stretch marks has been disappointing. Only a few treatments have had any success in clinical trials: 
- Centella, a medicinal herb, is the active ingredient in several creams that have been tested. Centella has been used in South Asia for hundreds of years to treat skin conditions like leprosy and eczema. A few small studies find that it may reduce stretch marks by about 20 to 30 percent when messaged into the skin.
- Bitter almond oil combined with massage reduced stretch marks a bit in one study, but there was no improvement when the oil was used without massage.
- Hyaluronic acid cream may improve the elasticity of the supporting structures of your skin by stimulating collagen production. A few small studies suggest that it may help reduce stretch marks.
- Tretinoin is a derivative of vitamin A that can be used as a skin cream. It has shown success in treating stretch marks from other causes, like rapid weight gain. Vitamin A is not safe in pregnancy, because it has been linked to birth defects, so this treatment is out during pregnancy or breast feeding. It might be an option for reducing the long-term effects of stretch mark scarring after pregnancy and breast feeding.
- Other studies have found no effect on stretch marks using coca butter, olive oil, or vitamin E.
Bottom Line on the Belly Mask
As of now there is no evidence to support the use of a belly mask or the active ingredients, aloe vera or propolis. In fact, there is really no strong evidence to support any method of preventing stretch marks. For most women, they are an unavoidable risk of pregnancy. [2,3]
Another problem with the mask is that it just lays on your belly. Massaging a skin care product into your skin seems to be a key component for effectiveness. The massage increases blood flow to your skin which may help prevent skin damage or help promote healing. Finally, stretch marks on your breasts are almost as common as stretch marks on your belly, and a belly mask will do nothing for them. [2,3]
You may be at higher risk for stretch marks if you had them in a previous pregnancy or if you have a family history of them. You may also be at higher risk if you are overweight before pregnancy, gain too much weight during pregnancy, or give birth to a big baby. 
Using the belly mask may help relieve some of the itching of stretch marks. It can’t hurt, but there may be better options for reducing your risk of stretch marks. The strongest evidence is for a cream that contains centella massaged into your skin and breasts. There are several over-the-counter options, but I would suggest asking your pregnancy care provider for advice. Getting regular exercise and eating a healthy diet that prevents excess weight gain are also important. [2,3]
- Hatch Collection, Belly Mask.
- British Journal of Dermatology, Stretch marks during pregnancy: a review of topical prevention.
- American Academy of Dermatology, Stretch marks: Why they appear and how to get rid of them.