An average of one set of Siamese, or conjoined, twins is born in the UK each year. Conjoined twins are created when a fertilised egg attempts to divide into identical twins, but fails to split completely.

The fastest-ever operation to separate conjoined twins lasted 45 minutes. The operation took place earlier this year at Great Ormond Street Hospital when Professor Lewis Spitz and fellow surgeon Edward Kiely operated on twin babies Zainab and Jannat Rahman, who were joined from the middle of the chest to the abdomen, and shared a liver.

Other conjoined twins who have survived the operation to separate them include Eman and Sanchia Mowatt, who were joined at the spine. They were separated in September 2001 during a 16-hour operation in Birmingham, even though there was a risk of paralysis because they had fused spinal cords.

Not every case ends so successfully. In 2000, Rosie Attard died after surgeons in Manchester separated her from her sister Gracie to save Gracie's life. Their Maltese parents, Michael and Rina, who felt that their daughters' fate should be left in the hands of God, fought a court battle to prevent the operation from going ahead, but lost the case. Two years on, they believe that it was God's plan to allow the operation to go ahead to save Gracie.

Sharing a heart, Natasha and Courtney Smith from St Albans, Herts, would not have survived surgery, according to doctors and both, sadly, died last May.

The world's oldest Siamese twins, Masha and Dasha Krivoshlappova, died earlier this year at the age of 53. They were taken into state care in Russia as babies and subjected to gruelling experiments but managed to survive the ordeal. Their adulthood in Moscow was also unhappy. Dasha started drinking heavily, which left her sister also feeling the effects of vodka because of their shared circulatory system.

Hussein and Hassan Salih, born in Sudan, who were fused together from the chest to the hip and shared a liver, were successfully separated. Now, 16 years on, they are strong and energetic, and have just taken their GCSEs in west London.