Stretch marks are lines on the skin that start off as raised red lines. They then turn purple, before slowly fading into flat silvery streaks. The medical name for this type of mark is stria. Stretch marks happen when the skin is stretched a lot over a short period of time, such as during pregnancy.

The skin is made up of three parts: the epidermis (outer layer), the dermis (middle layer), and the subcutaneous (inner layer), where fatty tissue is stored. The dermis helps to support the skin and keep it firm. It also contains blood vessels that supply skin cells with nutrients, and nerve cells that give us feeling. Stretch marks occur in the dermis.

The dermis is made up of strong fibres that connect to each other and allow the skin to stretch as we grow. Over time, these small fibres become thinner and less elastic (stretchy), and some of them break. Where the fibres break, the skin becomes thinner, and these areas show as stretch marks. When the skin is stretched thinly, the blood vessels under the surface show through, which is why stretch marks look reddish in colour.