It is amazing what lengths people will go to to deny that human life starts at conception. You hear arguments like, "What about sperm/egg cells, then? That's human life, too, isn't it?" or "But every human cell is alive. Should we avoid rubbing our skin because our epithelial cells might come off?"
That life starts at conception is proven beyond doubt by science. With the fusion of an egg cell and a sperm cell, the resulting zygote possesses all the genetic material characteristic of a unique human person and is also programmed to develop further into a fully-developed human being. Sperm and egg cells only contain half the genetic material of the parents; epithelial cells, while containing all the genetic material of a person, are not programmed to develop further into unique persons.
It really is as simple as that!
It is interesting that Christianity has not always argued that life begins at conception since in the ancient world the scientific basis for this was not understood. But it has always forcefully held that all kinds of abortion was morally wrong. Now we know why. Science has proved Christian faith right.