Surgeons have successfully separated 4-month-old twin girls joined at the breast and abdomen after an operation lasting almost six hours. An An and Xin Xin were on Tuesday in stable condition at the intensive care unit of Shanghai Children\'s Medical Center, where doctors say they will need help from breathing apparatus for some time. \"The girls have separate hearts, so the connection was not complicated,\" said Chen Qimin, one of the doctors involved in the surgery. The operation began at 9 am on Monday and involved four groups of doctors working simultaneously for almost six hours. \"As well as separating the girls\' hearts and livers, we also needed to reshape their breast bones and remodel their breasts with titanium-alloy plates,\" Chen said. When An An and Xin Xin were born in April this year, they were connected by the liver and a sac surrounding the heart. They were transferred to Shanghai Children\'s Medical Center two hours after their birth. \"At that time they weighed a combined 4.9 kg, and they needed to gain more weight before we could operate,\" Chen said. \"They weighed 10 kg (on Monday) after several months of comprehensive nursing in the center.\" The twins\' mother, a 39-year-old woman from Zhejiang province who declined to be interviewed, discovered her babies were conjoined during a prenatal check 18 weeks into her pregnancy. After learning that the twin girls were only connected at the liver, chest bone and heart sac, she decided to go ahead with the pregnancy under doctors\' advice. The Shanghai Children\'s Medical Center has previously separated two pairs of conjoined twins. \"Compared with the previous two, this is the first time a pair has been diagnosed during prenatal tests. We closely observed the twins during the pregnancy,\" said Liu Jinfen, president of the center. He added that research will be carried out to better detect congenital deformities at the early stage of pregnancy. Conjoined twins are rare, with the deformity occurring once in every 50,000 to 100,000 births worldwide. China has a high incidence of congenital deformities, and each year about 800,000 to 1.2 million newborns are affected by birth defects, such as congenital heart disease, cleft lip and palate and digestive tract problems.