BIRMINGHAM - In what is believed to be the first operation of its kind, doctors revealed yesterday that they had performed a heart operation on a baby while it was still in its mother's womb. Two years ago they unblocked a heart valve by passing a wire through the mother and into the baby's heart during the three hour operation at the Birmingham Women's Hospital. The mother was six months pregnant at the time and three months later a baby boy was born alive and well. Doctors kept the operation secret because of uncertainty about the extent to which it had helped the baby's condition.