To understand stretch marks we need to understand the science behind our skin. As the name suggests, stretch marks are caused by skin stretching. This can be due to pregnancy, weight gain, growth in adolescence, and even working out. However, your chances of developing stretch marks can vary based on your own genetics. Our skin produces a variety of hormones, proteins, and acids that are designed to help our skin regenerate and stay healthy. This includes producing cortisol, collagen, and hyaluronic acid. As we age, our bodies might not produce as much of these core building blocks for our skin. This is why you are more likely to develop stretch marks as you age than when you were younger. One common misconception is stretch marks only happen when you gain weight. However, teenagers can develop stretch marks when going through a growth spurt, and adults can gain stretch marks when training certain muscle groups like the biceps.