And how did this "ovum" come to be?
It was produced by the mother's ovaries in a process that randomly varied her own genes to produce a genetically distinct reproductive cell.
The point however is that every step of the process involves living organisms and living cells -- the sperm and ovum, the ovaries and testes, the father and mother, the sperm and ova that combined to make them, the ovaries and testes of their parents, and so on and so on. Live doesn't have a beginning except the first emergence of living things on this planet. The embryo is not a beginning but a stage in a continuum; it emerges from life and will go on to engender more living cells by cell division (assuming it's viable, of course).
You cannot use biology to justify an anti-choice position. Any attempt to do so amounts to substituting the wrong question for the right one, for example trying to claim that an embryo at conception is a person because it is "human life."
Well, it certainly is human life, but we destroy human life all the time and don't call it murder, suicide, manslaughter, accidental death, justifiable homicide, or any other term that applies to killing a person.
Is the embryo at conception a person? That's what you need to answer. That it is "human life" is purely irrelevant.