But Jewish tradition defines human life somewhat differently. Jewish tradition depends heavily on scientific observation when deciding relevant issues. Conjoined twins start from a single fertilized egg - in fact, such twins are probably not formed until at least three weeks after conception, when the developing embryo has failed to split completely. It seems illogical to suggest that one soul is formed at conception, but it becomes two separate souls three weeks later. So Jews place emphasis on the continuum of human life.
Life has no precise beginning, but gains in importance during human development.
Even human life is not absolute. Abortion is practised in most of the Western world for genetic defects such as Down's syndrome. Yet abortion is anathema to orthodox Jews. But Jewish law certainly allows it during early human development if it is threatening the mother's life, which is seen as more important to protect. It may even countenance abortion where a mother's mental health will be so damaged as to affect her life.