The unborn child is a patient, just as that child's mother is. A doctor treating a pregnant woman has two patients, and any ethical doctor will be concerned for the well-being of both.
But even if that were not so, it's always ethical to lie, or commit any other minor wrong, in order to prevent a much greater evil. It is certainly ethical to lie in order to protect an innocent person from someone who seeks to unjustly take that person's life.
If I knew that there was someone who wanted to murder my neighbor, and that would-be killer asked me if I knew where that neighbor was, I would lie, if necessary, in order to obstruct the would-killer from finding his intended victim. It has nothing to do with whether or not any kind of professional relationship exists between me and the neighbor. If I were to tell someone where my neighbor was, knowing that the person to whom I was telling that intended to murder my neighbor, this would make me a willing accessory to that murder.
A doctor, telling a pregnant woman that her child has a condition such as Down's syndrome, knowing that that woman intends to murder the child if the child has such a condition, would similarly be a willing accessory to that murder.
