SHANGHAI (AFP) - A Chinese hospital said Tuesday it had successfully separated four-month-old conjoined twin girls in a challenging six-hour operation. The twin girls, nicknamed An An and Xin Xin, were born in April with their liver and pericardium the sac that contains the heart connected, Chinas official Xinhua news agency reported. The two babies are now in a stable condition and are being closely watched in the intensive care unit, a spokeswoman for the Shanghai Childrens Medical Centre, who declined to give her name, told AFP. The centres director, Liu Jinfen, told Xinhua that four groups of doctors worked to separate the girls in the surgery, which began on Monday morning. We separated their hearts and livers, reshaped their breastbones and remodelled their breasts with titanium-alloy plates, Liu said after the operation. The twins entered the medical centre shortly after their birth in April, as doctors wanted them to gain weight before any operation. The two weighed a combined 4.9 kilograms (10.8 pounds) at birth, and 10kg just before the operation. The operation is challenging, but we made it, Liu said, adding that the girls might be reliant on a machine to help them breathe for some time to come. The twins will stay in the intensive care unit for at least a month. Their mother, whose name was not given, discovered the condition of her babies during a pre-natal check, but decided to keep the twins after doctors told her about the procedure. Shanghai is recognised as having among the best healthcare in China and the spokeswoman said it was the hospitals third successful procedure to separate conjoined twins. Up to 1.2 million newborns are affected by birth defects in China every year, the Shanghai Daily said. China, with the worlds largest population at more than 1.3 billion, had over 16 million births in 2009, the latest figure available.