According to Libby Downling, senior clinical advisor at Diabetes UK: "Ten to twenty per cent of women living with PCOS will go on to develop diabete di tipo 2 at some time. This is related to insulin resistance, and high levels of insulin circulating in the blood."

Researchers don't yet fully understand how or why PCOS affects women's insulin resistance. Women with PCOS are more likely to be overweight, but even when this is taken out of the equation, women with PCOS are more resistant to insulin. Many women with PCOS produce more insulin and clear it from the body more slowly through the liver. Insulin resistance may be linked in part to a problem with the insulin receptor.

And, as a result: "Women with polycystic ovary syndrome (PCOS) are more insulin-resistant than weight-matched women who do not have polycystic ovary syndrome," explains consultant endocrinologist Professor Stephen Franks, a spokesperson for charity Wellbeing of Women e Professor of Reproductive Endocrinology at Imperial College.

"It's that insulin resistance which means that the pancreas is having to produce more insulin to keep the blood sugar normal. Eventually, it fails to compensate for that, and you develop type 2 diabetes. So insulin resistance is a predisposing factor for type 2 diabetes," he adds.

Weight-related risks for PCOS and diabetes

However, he says, it's important to be aware that weight also plays a role in the development of type 2 diabetes.

"The increased risk of type 2 diabetes in women with PCOS is slight in lean women, but it's considerably higher if you're obese," Franks explains.

"In a northern European study, lean women with PCOS didn't seem to be at significantly increased risk of type 2 diabetes. But if you were overweight, it was something like three to four times the risk compared with overweight and obese women without PCOS. So it's the interaction between PCOS and obesity which is the critical thing," he adds.

Despite this, PCOS charity Verity points out that developing type 2 diabetes "can sometimes occur regardless of weight and it is believed women with PCOS are up to 40% more likely to develop diabetes than a woman without the condition". In Southeast Asian women with PCOS, they add: "This number can be as high as 50%."

When it comes to prevention, Franks says the first step is to assess the risk of diabetes in women diagnosed with PCOS.

"If we see a woman who's been diagnosed with polycystic ovary syndrome, whether she's overweight or not, we would want to find out a little bit about her family history," he explains. "If you've got a family history of type 2 diabetes, you're more likely to have it yourself, so that's a risk factor."

Beyond that, he adds, the key thing is minimising weight gain in women who have PCOS, and encouraging a perdere peso in those who are overweight or obese - even if they don't have type 2 diabetes or blood sugar abnormalities.

"When we see women with polycystic ovary syndrome in the clinic, particularly if they're overweight, we keep an eye on them even if they haven't got any sign of type 2 diabetes, and monitor their blood sugars from time to time," Franks explains.

"If you have impaired glucose tolerance - in other words, your glucose levels go up a bit more than they should after eating, but don't reach the levels that you see in type 2 diabetes, that means your pancreas isn't quite keeping pace, so then we'd intervene more," he adds. "We'd be more keen that that woman loses weight and is more closely monitored - whereas if the glucose levels are totally normal, we wouldn't be quite so worried but would keep monitoring her."

In short, Franks says: "It's a good idea if you've got polycystic ovary syndrome, to try to keep as slim as you can." But, he adds: "Having said that, it's not all that easy, and there's some evidence to suggest that women with polycystic ovary syndrome are more prone to obesity as well. We've still got a lot to learn there."

For women with PCOS and diabetes, the diabetes would be treated in much the same way as in patients who have type 2 without PCOS - usually using a combination of lifestyle changes and drugs to improve glucose levels.

"Treatments for PCOS and diabetes both include maintaining a healthy weight and being physically active, as well as prescription of the drug metformina, which is also often prescribed to people with type 2 diabetes," Downling explains.

It's also worth knowing that developing type 2 diabetes shouldn't affect your existing PCOS symptoms, Professor Franks adds.

"It's more a question of being overweight making the symptoms of polycystic ovary syndrome worse, as well as increasing your risk of type 2 diabetes," he explains. "So if you're overweight, you're more likely to have unwanted body hair and problems with your periods. But the good news is, if you lose weight, you can reverse that to a large extent."