Doctors at a London hospital said Tuesday night they had conducted what they believe are the world's first successful heart operations on a baby still in the womb. Lindsey Allan, the doctor in charge of the fetal cardiology unit at Guys Hospital, said the team there had corrected a heart valve defect in an unborn boy using a balloon catheter guided into place by ultrasonic sound. ``As far as I know this is the first successful operation of its kind,'' she said. She said two operations were carried out on the fetus during the mother's pregnancy, one at 31 weeks and the second at 33 weeks. Ms. Allan said the baby was born prematurely on Jan. 4 _ one week after the second operation _ and underwent surgery for the third time a few hours after birth. She did not say why there had been a delay in disclosing details of the operations. ``The child is still quite seriously ill, but we have given him a chance he would not otherwise have had,'' Ms. Allan said. She said the parents, who did not want to be identified, lived outside London. The operations involved inserting a catheter down a hollow needle directly into the chest wall and heart of the fetus. Once in place, the balloon was dilated to open the heart valve. A catheter is a slender tube inserted into a body passage, vessel or cavity. Its normal uses include passing fluids and making examinations. Ms. Allan said the team that pioneered the technique included herself; Michael Tynan, professor of pediatric cardiology; and Darryl Maxwell, director of the hospital's fetal medicine unit. She said there were always ethical obstacles to this type of operation. ``I am not sure that we should be meddling sometimes. There is always discussion as to whether it is a reasonable request to make of the mother. On the other hand, one knows that the alternative is almost universally fatal,'' she said.