Some Muslims argue that abortion is permissible if the fetus is younger than four months (120 days). They quote a statement from the Prophet (s) that refers to a human being starting as a fertilized ovum in the uterus of the mother for forty days, then it grows into a clot for the same period, then into a morsel of flesh for the same period, then an angel is sent to that fetus to blow the Ruh into it and to write down its age, deeds, sustenance, and whether it is destined to be happy or sad.
Some Muslims argue that the only case when aborting a fetus, before or after 120 days, is allowed in Islam, is when a medical situation threatens the life of the mother, leaving only two options, to let either the other or the fetus survive, but not both. Scholars argue that such a case can only be determined by a specialist, trusted and committed Muslim doctor. They argue that the mother can have other children, whereas the child cannot make up for losing the mother.
However, there is one exceptional situation. If, say the jurists, after the baby is completely formed, it is reliably shown that the continuation of the pregnancy would necessarily result in the death of the mother, then, in accordance with the general principle of the Shari'ah, that of choosing the lesser of two evils, abortion must be performed. The reason for this is that the mother is the origin of the fetus; moreover, her life is well established with duties and responsibilities, and she is also a pillar of the family. It would not be possible to sacrifice her life for the life of a fetus which has not yet acquired a personality and which has no responsibilities or obligations to fulfill.
Allah Almighty knows best.
http://www.islamawareness.net/FamilyPlanning/Abortion/abortion3.html
