BEYONCE singer says the pre-eclampsia, also known as toxemia, put her on bed rest.

Beyonce is certainly no stranger to breaking boundaries - she's the most nominated woman in the history of the Grammy Awards, as well as being one of the world's best-selling music artists of all time.

Now, she's made waves with her September cover of Vogue. It's rumoured she was responsible for choosing Tyler Mitchell to shoot the pictures, making it the first time in the magazine's history that a black photographer has done the cover.

To accompany the striking photos, Beyonce told her story in her own words, covering everything from body acceptance to her ancestry. The star rarely gives interviews, so it was surprising that she was so open about the difficulties during pregnancy.

She says: "I was 218 pounds (15.5 st) the day I gave birth to Rumi and Sir. I was swollen from toxemia and had been on bed rest for over a month. My health and my babies' health were in danger, so I had an emergency C-section."

Toxemia is another term used for pre-eclampsia - here's everything you need to know about the condition unique to pregnancy.

What are the symptoms?

The two earliest signs of pre-eclampsia are high blood pressure and protein in your urine. Obviously these aren't exactly things you can diagnose on your own, but the NHS says they should be picked up during antenatal appointments.

Further symptoms could include; swelling of feet, ankles, face and hands, headaches, vision issues and pain below the ribs.

Who is at risk?

Pre-eclampsia normally occurs around the second half of the pregnancy, or just after the baby is delivered.

According to the NHS, it's a relatively rare condition that affects up to 6% of pregnancies, with severe cases occurring in 1-2%.

Chances of developing pre-eclampsia are heightened if you already suffer from things like diabetes, high blood pressure, kidney disease or lupus. Chances are also increased if you're expecting multiple babies - like Beyonce who gave birth to twins.

How is it treated?

If you're at risk of developing pre-eclampsia, the NHS says you might be advised to take low-dose aspirin every day from your 12th week until the baby is born. Strangely, doctors don't know the exact cause of pre-eclampsia, but believe it is to do with something going wrong with the placenta.

It could perhaps be due to this lack of understanding around the condition that means it's difficult to treat. For the majority of sufferers, toxemia is cured only when the baby is delivered. Symptoms have to be closely monitored to ensure that they're not worsening, and in many cases labour is either induced or the woman has a C-section like Beyonce.

Another suggestion the NHS makes is taking medication to lower your blood pressure before giving birth.

Most women with pre-eclampsia will deliver healthy babies and fully recover, however if it's undiagnosed it can lead to eclampsia, a serious condition that can be life-threatening to the mother and baby.