This is a good question.
The answer is provided in the Upanishads, and elaborated in the Brahma Sutras by the Brahma Sutra commentators.
Sin and merit is nothing other than punishment and reward by Brahman (God) for actions that displease him versus actions that please him:
Kaushitaki Upanishad - For He [Brahman] makes him, whom He wishes to lead up from ...
Both a fetus and cancer are parasites
This question must be a joke. You're comparing a human baby, with a soul inside it, to a disease.
Abortion is a sin because there is a Jivatma in the fetus, which is a living human being ready to come into the world to carry out his karma and dharma. The Jivatma enters the woman at the time of conception when the man ...