To put it simply, stretch marks form when the middle layer of skin, the dermis (below the epidermis) is stretched to breaking point. This layer splits and reveals the layer and blood vessels beneath, hence the purple / pink colour.
There are multiple factors that can cause the dermis to break:
• Growth spurts. If the body goes through a period of rapid growth, for example, during puberty, the fibres that make up the structure of the skin can become over-stretched and eventually pull apart.
• Pregnancy. In this case, and especially after the sixth month, the skin is subjected to both rapid stretching and to hormonal changes. The hormones are designed to make pelvis ligaments softer in preparation for labour, and yet they have the same effect on the dermal fibres, making stretch marks more likely.
• Rapid weight gain. Stretch marks can occur whether the weight gain is due to increasing volumes of fat or muscle, so bodybuilders can be prone to getting stretch marks. The only way to reduce the likelihood of getting stretch marks is to ensure that any weight is gained gradually.
• Excess cortisol. Cortisol is produced by the body, but when there is too much, it can reduce the amount of collagen present in your skin. Collagen is essential to the skin’s ability to stretch. Some medecines for eczema act in a similar way to cortisol and can cause stretch marks.