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You may have a years-old bra, now slightly stretched or discoloured, that you love for how comfortable it is. You might also have a bra you've been unable to wear more than once because it doesn't feel right - maybe the wires poke into your skin, or there's too much padding.

Can your bra cause breast cancer?

There are a whole host of myths out there when it comes to breast health, even suggesting that wearing bras causes cáncer de mama.

Amy Sanchez, Senior Research Associate in Biomechanics with the Research Group in Breast Health at the University of Portsmouth says that wearing a well-fitting bra has no known negative consequences.

She explains: "We often get asked about cancer and bras, and existing research shows no link between bra use and breast cancer."

However, she warns that while bras don't cause breast cancer, that doesn't mean that they can't impact your health.

"Wearing an ill-fitting bra can cause poor posture, espalda y neck pain, shoulder grooves leading to numbness in the fingers, and a lack of self-confidence," says Sanchez.

Tracey-Jane Hughes, founder of Bra Lady agrees that wearing an uncomfortable bra can have an affect on body confidence and self-esteem.

With wear, bras stretch and don't provide the same support they had when you first bought them. Your body also changes over time so it's a good idea to buy new bras regularly.

"A bra will last for about six months," says Hughes.

There's a growing trend to turn to non-wired soft-cup bras, fabric 'bralettes' or not wearing a bra. In particular there has been an increase in the number of bralettes on the market.

So is an underwire always necessary?

Sanchez says: "As far as I am aware there has been little research done specifically on bralettes. We have, however, tested several non-wired bras and these can be effective at supporting your breasts if constructed well.

There are many different materials and techniques used in the bra industry and it is no longer essential for a bra to have underwire to provide good support."

Sanchez explains that wearing no bra - or a low-support bra - can cause your skin to stretch beyond its elastic limit. This could lead to permanent deformation and premature sagging of the breast.

"There is also plenty of research to show that wearing low breast support leads to increased dolor en el pecho and reduced movement efficiency during exercise or physical activity," she adds.

Body shape can make all the difference when it comes to buying a bra.

Hughes says that if you have larger breasts you may not get the support you need from a bralette. You are likely to end up with neck ache or back ache because it's not supportive enough.

Fit is just as important in bralettes as in traditional bras.

"An ill-fitting bra is an ill-fitting bra - it doesn't matter whether it's got wires in or not," she warns. "Soft-cup bras are becoming more popular. But if you've got an ill-fitting soft-cup bra, that will still do as much damage to your breasts, back and neck as a underwired bra."

You will have you own criteria for your bra, including style, support and comfort. The Research Group in Breast Health advocates a 'best-fit' approach to finding the right bra, using five key criteria.

Sanchez advises: "If a bra does not meet all these criteria try going up or down a size."

The underband is crucial to getting the perfect fit. Many wear the underband too loose and the cup too small.

Hughes explains: "You wear it around the smallest part of your back. You might be wearing them too high - so your breasts aren't being supported.

"Make sure your breasts are enclosed in the cup. We all have one breast bigger than the other so we always fit the larger breast which can be one or two cup sizes different."

If you're lucky enough to find your 'perfect bra' don't forget that it might not be suitable for all occasions. Breasts change during the month - and even during the day. Hormones can make your breast size fluctuate during your monthly cycle, up to one to two cup sizes. What you need from your bra also changes at different times.

Sanchez says: "Consider wearing different bras for different activities, as support level, strap configuration, adjustability, and style requirements are likely to change between activities. Invest in a well-fitting bra if you are having discomfort or embarrassment associated with your breasts - even if it's only worn at certain times of the month, or for specific activities."

It can be difficult to know if a bra works for you until you've tried it out for a few hours. However, you can check if it's fit for purpose by trying it out in the fitting room in the way that you would normally wear it. This helps you make sure that it isn't too tight, gives you support and lets you move in the way you need to.

Hughes suggests: "Try your top on. Does it look good? Does it feel good? Jump up and down, swing your arms around. Make sure your bra is fit for the purpose you need it for. If it is a sports bra, do some of the things that you would do if you were doing that sport.

If you're sitting down in a job all day, sit down on a chair to try it out because our bodies are in a very different position from standing to sitting. Try it out as you would wear it every day. If it's not comfortable, it's definitely not right. If you've got any doubt at all, don't get it."