My wife recently had an X-ray. After that, she discovered that she is pregnant. Some of the doctors said that she must abort the fetus because there is a risk that he will have serious problems like to be mentally or physically handicapped. And the others said that she must not do so, because if there was any problem, he would have already died. We are afraid that we might kill a human being and at the same time that he could be an abnormal human being if he lives. What must we do?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Abortion is prohibited in Sharia as it is murdering a soul which might (if left alive) worship Allah, believe in Him and praise Him. The prohibition becomes greater if the fetus is aborted after it has completed four months, i.e. after the soul is secured in it.
Scholars mention the following as a case where abortion could be done. If some trustworthy doctors make an accurate report proving that by not aborting the fetus his mother's life will be endangered, then abortion becomes lawful.
As for what you described, i.e. the fear of getting a handicapped child, this is not a legal reason for committing this abominable crime. We advise that you be patient and ask Allah to give you a healthy, good and pious child. Your patience and your acceptance of what Allah has predestined for you may be a cause for you and your wife to be rewarded by Paradise. Again what the doctors said might only be speculation that could be wrong. Any way, you are asked to be patient and be pleased with what Allah has given you. Indeed, this life is full of trials and discomfort.
But in the Hereafter one will be rewarded for any good deed he has done.
We ask Allah to guide us and you to doing what pleases Him and to being happy with whatever He gives us.
Allah knows best.