I'm unfamiliar with "zygote" - Is that not an ovum that's been fertilised by a sperm (thus the eventual foetus)?
How did you come to that conclusion? Does the amalgamation of a sperm cell and an ovum also include the immediate inception of a soul? I happen to think that this is the wrong question, regardless of the answer. I would be more interested to find out how we can quantify suffering in the matter.
"All thinking people recognize a painful conflict of rights and interests in this question, and strive to achieve a balance. The only proposition that is completely useless, either morally or practically, is the wild statement that sperms and eggs are all potential lives which must not be prevented from fusing and that, when united however briefly, have souls and must be protected by law. On this basis, an intrauterine device that prevents the attachment of the egg to the wall of the uterus is a murder weapon, and an ectopic pregnancy (the disastrous accident that causes the egg to begin growing inside the Fallopian tube) is a human life instead of an already doomed egg that is also an urgent threat to the life of the mother." - Christopher Hitchens
Perhaps I wasn't clear. I meant to say that the idea of observing the right to life for all potential clusters of reproductive cells is a mad one if you try to uphold it. Shining Raichu made a salient point - is a sperm cell by pro-life logic not a potential person?
People who think in this way are trying to create a rule - I applied this rule thoroughly (hypothetically) and am pointing out that it does not work.