For many women, they're the red, white or sometimes brown badges of bearing children.
But while pregnant women are the group with whom we most frequently associate stretch marks, a larger percentage of the population is not immune. Weight gain can bring on stretch marks, too - men and women alike can end up with unsightly reminders of heavier days. Even people who went through sudden extreme growth spurts during adolescence often have marks around their joints or on their backs. And bodybuilders sometimes get them on the upper chest and shoulders.
Sometimes. That's part of the frustration - and the mystery. Some people get them, and some people don't, even under similar conditions.
Whatever the cause, the main sticking point is they never really go away, and certainly not on their own. The topic - especially for women - is a delicate one, akin to discussing personal finances or face-lifts. For some, the problem is more than skin deep.
Anne Nguyen, 32, says she felt like "damaged goods" when pronounced stretch marks appeared after she had her first two children.
"It has affected my confidence, and I never wear a bathing suit and avoid situations where I would have skin showing," she says.
"I hardly get in the pool with my kids, and they don't understand why.
What frustrated her is that she has always been physically fit. During pregnancy the weight gain came rapidly and stretched her skin.
"It's embarrassing, and it's hardly ever discussed," she says.
"I know there are other problems that are more traumatic and life-threatening, but stretch marks do affect a person's emotional state. Knowing it will never go away affected me."
Many dermatologists say the cause is literally a stretching of the skin, to the point where connective tissue breaks down. Others assert it's not entirely a matter of stretching but of hormonal variations during extreme changes to one's body. Almost all believe there's a genetic component.
Various laser and even radio frequency treatments can minimise the appearance, but if you're prone to getting stretch marks, you're pretty much stuck with some permanent road maps to your skin's past.
Nguyen has undergone several laser treatments with Dr John Tang at the Rejuve nonsurgical cosmetic care clinic in California.
"We can reduce the appearance, but you can't ever really eliminate them," Dr Tang says.
He adds that hormones can be a factor.
"On a young healthy person, stretch marks are normally caused by rapid increase or decrease in weight," he says.
"When we get older, the stretch marks we have often look worse due to hormonal loss, especially for women because of loss of oestrogen and growth hormone, which controls elasticity and collagen production."
Dr Richard Nolan, of aesthetic medicine specialists Laser Skin Source, likens human skin to a delicate fabric.
"If you push your finger through the fabric, you stretch out the fibres, and it never will go back," he said. "But it's not the same for everyone.:
The marks occur in the dermis, the resilient middle layer of the skin that helps it keep its shape. They most often show up in areas of your body where fat is stored - the stomach area, breasts, upper arms and thighs.
Many dermatologists say hormonal changes and genetics influence the skin's capacity to withstand stretching. And some researchers have artificially created stretch marks on normal skin by applying strong topical steroid creams, suggesting a hormonal cause. Also, sun exposure, smoking and diet can affect connective tissue, Dr Tang says.
"And there's obviously a genetic predisposition," says dermatologic surgeon Dr Min-Wei Christine Lee.
"It tends to be if your mother and grandmother had them, you'll get them, too. It's so variable. For some people, even a slight bit of weight gain will do it."
Not all stretch marks are equal. Some appear reddish, while some are purple or whitish. Some start out pink then change over time to a silvery appearance, which indicates the most damage. And they vary with different skin tones and types. Nolan says Asian skin seems to be particularly prone to the development of the marks.
The most popular procedures are various laser therapies to stimulate new growth of collagen and elastin. Tang uses a radio frequency device in conjunction with lasers. A study in the journal Dermatologic Surgery showed that radio frequency combined with pulsed-dye laser treatment provided "good and very good" improvement of the appearance of stretch marks in 33 of 37 patients, but doctors say more research is needed.
A relatively new treatment is fractional laser resurfacing, using scattered pulses of light on one "fraction" of the mark at a time over the course of several visits. This creates thousands of microscopic wounds, and the skin responds by producing new collagen and tissue at the body's outer surface.
As to over-the-counter creams, Lee has not found them to be very effective on their own.
"Anti-stretch-mark creams sound magical, and they may be helpful combined with other treatments," she says.
"But they're only really good moisturisers. They may help keep skin from looking worse and improve skin texture, but they're not a cure."