What is the Islamic opinion on the time when abortion is permissible?

Allah Ta ala says in the Quran: Life is from a decree of my Lord. (Sura 12 verse 85) the above verse is self explanatory on the sanctity and significance of life. Generally every decree is from Allah Taala but the honour of attribution increases its sanctity and demands its highest level of reference. This command is mentioned in many verses of the Quran: Allah  Ta ala says: "Whosoever has spared the life of a soul, it is as though he has spared the life of all people. Whosoever has killed a soul, it is as  though he has murdered all of mankind." (sura 5 verse 32) The fact that  every one soul is equivalent to all of mankind clearly indicates the value of every life.

The above should suffice to express the sanctity of the Rooh (life). It is not permissible under any circumstances to destroy a rooh. The jurists have mentioned the life enters the foetus upon 120 days. From then onwards the foetus is a living human being.

Under no circumstances will it be permissible to abort the child. Allah Taala in his infinite knowledge knows the circumstances, inspite of Him Being Most Merciful, He has endowed the foetus with life. Can our deficient mercy to abort the child be compared to Allah Ta ala s Infinite Mercy to sustain the child. The heights of His Infinite Wisdom cannot be reached or comprehended. The importance of the biggest problem in the circumstances cannot be equivalent to the importance of the Rooh (life). It is too dear to be discarded. Before 120 days too, it is not permissible to abort the child.
However, it may be permissible to abort before 120 days in the following instances:

1) Rape
2) Incest
3) Sexual interference with the mentally retarded
4) Foetal deformity for e.g.. anencephaly (no brain)
5) Foetal deformity for e.g.. congenital rubella health.

And Allah Taãla Knows Best.

Article taken (with Thanks) from Islam.tc