All perfect praise be to Allah, The Lord of The Worlds and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.

Allah, The Exalted honored human for He Said (What means):" Verily we have honoured the Children of Adam." [Al-Isra'/70]. One of the images in which he is honored is that He prohibited killing a human and considered this act as if a person killed all mankind and saving one life is similar to the rescue of all mankind, since He Said (What means):" whosoever killeth a human being for other than manslaughter or corruption in the earth, it shall be as if he had killed all mankind, and whoso saveth the life of one, it shall be as if he had saved the life of all mankind." [Al-Maidah/32].

Committing a crime over a fellow human is prohibited regardless of the age; whether this took place before or after birth. In this regard, Al-Imam Al-Ghazali (May Allah have mercy on his soul) stated: "The first stage of existence is the formation of semen-drop and its mixture with woman's vaginal fluid in which it is preparing for being alive and spoiling it is prohibited. In addition, the prohibition of making abortion or violating it in any form increases during it is transformation into embryo. At the time in which Allah  breath into it of His spirit, the prohibition of abortion increases the most." [Aihyaa' Oluom Addin vol.2 pp.51].

Therefore, violating the embryo by the mother is prohibited; however, the mother afflicted with cancer is permitted to abort her embryo if she was having radiotherapy or chemotherapy so long as it didn't exceeded four months and doctors were assure that this therapy might cause deformity to the embryo by which its life won't be stable. 

This was stated by the Board of Iftaa in its resolution No. (35) for 1413 AH as follows:" If it became four months old or more, then aborting it is impermissible, even if it was deformed, in case the doctors determined that it is likely to survive. However, if it jeopardizes the mother`s life, then it is permissible to abort it. Moreover, if it didn`t become four months old, and it was confirmed that its deformity will make its life unstable, then it is permissible to abort it with the consent of both spouses." And Allah knows best.