There are many religious arguments associated with abortion, arguing that sanctity of life and the commandment "you shall not kill", prevent abortion from being morally acceptable. However, this is dependent on when the ball of cells inside the mother can be defined as an individual human life, and not just an extension of the mother's body.Some people believe that life begins at contraception, for example Catholics, yet at this stage the fertilised ovum is just a clump of cells, and it is not even possible to define which cells will go on to form human cells, and which will form the cells of the placenta, surely this cannot be considered a human life? It becomes clear which cells are human cells when the primitive streak appears; however, people have argued that all body cells are human cells, but cutting off a limb is not considered murder.

In my opinion, this is a weak argument, as the fact that the human cells have the potential to grow into a human being is not taken into account, so an early stage embryo is in no way comparable to a limb, as a limb has no potential. On the other hand I do not think that just because cells are recognisable as human, it means that the embryo is a human being. Judith Javis-Thompson argued that although an acorn has all the potential to be an oak tree, there is a continuous growth from an acorn to an oak tree, and an acorn is not an oak tree.Similarly, an embryo is not yet a human person, but will slowly become one over time. I agree with this as I think it is inconsistent that an embryo should be an extension of the mother's body one day, and a human being the next.

The issue of when or if an embryo becomes a human life is central to all these arguments and to the abortion debate in general, as personal beliefs about the status of a foetus determine whether certain arguments have any significance.For example, if a person believed that human life does not begin until birth, then all religious arguments involving sanctity of life would be irrelevant. However, there are other issues involved in the abortion debate that focus more on the balance of rights between the mother and the foetus than on the definition of human life. For example, if the mother's life is in danger, most people would agree that the mother's rights to life come before that of the child, as the mother has dependants.

Yet, if a Catholic believed very strongly that a foetus is a human life and abortion is murder, then the mother would have the same rights to life as the foetus, and the mother may feel obliged to put her life in danger to save the foetus. Therefore, ultimately, even issues involving rights are dependent on the definition of human life and at what stage a foetus is considered to be a human.