Bras. It's not the kind of underwear women buy very often.

Buying a bra might seem simple but it is actually quite complex. Studies show that 70 percent of women buy the wrong bra.

According to Carol Mngomeni, an expert on bras at Temptations, eight out of 10 women wear bras that don't fit.

"The right bra can make your clothes look great, improve your posture and help you feel incredibly sexy," Mngomeni says.

"The wrong bra can have you scratching, pulling and fidgeting all day."

According to former beauty queen Uyanda Mbuli, who co-owns a clothing line called Diamond Face Couture, a bra must feel and look good.

"Bras that are too small in the cup compress boobs and force them to bulge out the sides. And cups that are too big can wrinkle and pucker, which won't look good under clothes," Mbuli says.

So what are the consequences of wearing badly-fitting bras?

Aside from issues of aesthetics and comfort, says Fatima Aziz, an osteopathic doctor, studies found that ill-fitting bras can push down on the ribcage, place pressure on the diaphragm and lead to breathing problems.

"A tight bra can also cause blood circulation problems and skin lesions. But by far the most common result of wearing a bra that doesn't fit well is suffering from back and neck pains, muscle tension and even headaches."

Aziz says this is because muscles in the shoulder and neck will tighten up to take the weight off the breasts if they are not properly supported.

"A bra keeps breasts supported while moving and can help prevent injury to breast tissue. A well-fitting bra is also important for a woman's posture, particularly if she has large breasts."

But with so many sizes, shapes and colours to choose from, buying a bra can be overwhelming. So where do you begin?

Mngomeni says the first step is to take a tape measure and measure the diameter of your ribcage directly under your bust.

The second step is to measure around your chest over the fullest part of your bust. And the third step is to determine your cup size by taking the difference between the two measurements.

"Subtract the measurement of step one from the measurement of step two," she says.

Mngomeni says if your chest measurement is an odd number, it's usually a good rule to round off to the next number.

There are different bras for different reasons, says Mngomeni.

After you have the right size, the next thing to think about is why you need the bra.

Mngomeni says bras come in a variety of fits and fabrics.

"There are different styles of bras designed for specific body types, fashions and activities. No one bra can do it all."