I think there is an important point that needs to be considered here. There is a categorical difference between scientific fact and philosophical argument. For instance, it is a scientific fact that the transition to rudimentary consciousness, and then more complex consciousness, occurs during the second trimester in most fetuses. Categorically, it is not a scientific fact, in the sense that it can be empirically measured, that personhood begins either before, during, or after this stage. Although I generally accept the statement that “human life begins at conception,” this assertion is grounded primarily in metaphysics; science probably cannot further inform one’s position on this particular problem.
I do accept the metaphysical view that conception represents the most natural point to delimit the start of a human life, although there are philosophical objections to this view. (Thomas Aquinas, for one, argued that human life does not begin until the fetus has a human soul–sometime during the second trimester, in his estimation. Incidentally, neurobiology has shown that this is the same period during which the gradual transition to consciousness begins. It seems that regardless of one’s opinion of Aquinas’ argument, the argument itself was based on reasonably accurate biological observations).
So the problem we run into here is a philosophical problem, not a factual problem. It is a fact that the genetic blueprint of the individual comes together (50% from each parent) shortly after conception. It is a fact that many zygotes spontaneously abort soon after conception. It is a fact that the infrastructure necessary for a transition to rudimentary consciousness does not develop until the second trimester. It is a fact that a full-term fetus has sophisticated neurobiology and a considerable degree of consciousness. It is a fact that humans share by descent many of the same biological pathways to development as other mammals, and indeed, non-mammal organisms. It is a fact that the human fetus has, on average, a greater capacity for subsequent intellectual development than any other organisms on Earth. These are all facts that a scientifically informed individual should have no problem accepting (more or less, although one could perhaps quibble on some minor details). The philosophical problem we are confronted with is the question, “What do these facts mean, fundamentally, about the nature of human life?” Unfortunately, although we are not entitled to our own facts, those facts necessarily must be interpreted through one’s own metaphysical filter.
In the end, I think the problem is one of differing philosophy, not differing fact. Unfortunately, few on either side of the debate will consider this true problem underlying the debate. I think more pro-choice folks should concede that a genetically unique individual truly is created shortly after conception. Likewise, more pro-life folks should accept that neural development sufficient for rudimentary consciousness does not take place till the second trimester. But abortion is more a theological debate than a biological debate, anyways. The Catholic who is sincere about taking the Catholic position on the issue needs to look to the church for guidance, not to the biological sciences. On the basis of facts alone, the biology argument is an un-winnable argument for either side. Those in the best position to use biological data for successful arguments are those folks who would like to see a compromise (i.e., abortions generally only in the first trimester except when the woman’s life is in danger).
I don’t know WHAT thread I was reading before… 
