Stretch marks appear when your skin is suddenly stretched. They occur in the strong middle layer of your skin (the dermis), which supports your skin's outer surface (the epidermis).
The dermis is made up of strong fibres that connect to each other and allow your skin to stretch as your body grows. However, if a part of your body grows rapidly over a short period, the fibres can become thin and over-stretched and some of them may break.
At the point where the fibres in the dermis break, tiny tears develop and the blood vessels that lie underneath show through. This is why stretch marks are a reddish colour when they first appear. Eventually, the blood vessels contract (shrink) to leave only the fat under your skin visible, and the stretch marks fade to a silvery white or grey colour.
Not everyone gets stretch marks. Some people may be more likely to develop stretch marks than others because their bodies produce a larger amount of a hormone called cortisol. Cortisol decreases the amount of collagen in your skin. Collagen is a protein in the skin fibres that helps keep it stretchy.
The main causes of stretch marks include:
These are explained in more detail below.
If you are pregnant, it is likely that you will develop stretch marks, particularly from the sixth month (second trimester) of your pregnancy.
Hormones that are produced by your body during pregnancy help to soften the ligaments in your pelvis so that they are more flexible when you come to deliver your baby. Ligaments are strong bands of tissue that connect joints. However, these hormones also soften the fibres in your skin, making you prone to stretch marks.
As your baby grows, you may develop stretch marks on your abdomen (tummy) as your skin is gradually stretched further and further. Stretch marks may also appear on your thighs and breasts as they get bigger and heavier.
Over time, most stretch marks will fade to lines that are pale or flesh-coloured after childbirth.
You may get stretch marks if you put on a lot of weight over a short period. The stretch marks may remain even if you lose the weight that you have gained. However, they should fade over time.
If you diet regularly, stretch marks can form as your weight goes up and down rapidly. If you are dieting, it is important to lose weight slowly and steadily so that your skin is not put under strain.
Bodybuilders and athletes can also sometimes get stretch marks as their muscles increase in size.
During puberty (the change from childhood to adulthood), the body tends to develop very quickly in growth spurts. As a result, males often get stretch marks on their shoulders and back, and females get them on their hips, thighs and breasts.
See the Health A-Z topic about Puberty for more information.
If you have close relatives who have stretch marks (such as your mother), you may be more likely to develop them yourself. Stretch marks may affect both male and female members of your family, although they are more likely to occur in women.
You may develop stretch marks if you use corticosteroid medicines, such as creams, lotions or tablets for eczema (a skin condition that causes itching and redness).
Corticosteroids work in a similar way to the hormone cortisol, which is produced naturally in your body. Medicines that contain corticosteroids can help ease the inflammation that is caused by skin conditions, but they can also decrease the amount of collagen that is in your skin.
Collagen is a type of protein that is needed to keep your skin stretchy. Therefore, the less collagen there is in your skin, the more likely stretch marks are to develop.
See the Health A-Z topic about Corticosteroids for more information.
Sometimes, stretch marks can be caused by rare underlying health conditions, such as Cushing's syndrome and Marfan syndrome.
Cushing's syndrome occurs when the body produces an excess amount of cortisol, the same hormone that may make some people more prone to developing stretch marks than others.
In Cushing's syndrome, your body produces so much cortisol that it can cause a variety of symptoms, including:
If you have Cushing's syndrome, you may develop noticeable stretch marks that are dark in colour.
See the Health A-Z topic on Cushing’s syndrome for more information.
Marfan syndrome is caused by a faulty gene that affects your body's connective tissues, including your skin. It weakens your body's tissues and affects their elasticity (ability to stretch), so that your skin is not as resistant to stretch marks as it should be.
If you have Marfan syndrome, you may develop stretch marks on your shoulders, hips or lower back. The condition also causes several other symptoms, such as curvature of the spine (scoliosis) and vision problems.
See the Health A-Z topic about Marfan syndrome for more information.
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