Question: If it is known that a baby will be born mentally disabled, is it justified for the mother to have an abortion?
    Answer: Islam gives great sanctity to human life. After conception, once the spirit has been blown into a human embryo thereby making the embryo a living soul, it cannot be destroyed unless on some medical grounds given by a doctor. Before this spirit is blown, the human foetus is actually not a human in the true sense; it is rather a ‘non-living’ mass of flesh and may be aborted. 
    Now the question arises as to how this period should be determined. For the moment, the only data we seem to have is a Hadith attributed to the Prophet (sws). It tells us that it is after 120 days (four months) that an angel comes and blows the spirit in the human embryo:

Indeed the creation of each one of you is brought together in the mother's  womb for 40 days in the form of a sperm,; then he is a germ cell for a similar period,; then an embryonic lump for a similar period; then there is sent an angel who blows the breath of life into him... (Bukhari, Kitabu’l Qadr)

    If this Hadith is authentic, then abortion after 120 days can take place only and only if a doctor advises it on medical grounds.
In the light of this, if the mental deformity of the foetus can be determined before 120 days, it may be destroyed. After this period, one should not even think of tampering with the process of birth unless of course the mother’s life is in danger.