In case you missed it on our home page, here is Douglas W. Kmiec's insightful essay on stem cells and church teaching. Among the questions Professor Kmiec addresses is where the line of "personhood" should be drawn.
Scientists and ethicists challenge the pro-life office’s proposition that the personhood line cannot be drawn better elsewhere. Many non-Catholics propose the point at which an embryo is implanted in the womb as one that can be easily demarcated scientifically. From a Catholic perspective, the proffered line might be argued to better coincide with Jeremiah’s reference to God’s knowledge of us “in the womb,” and even the statement of the pro-life office itself. After all, the pro-life office recognizes that the capacity of the embryo to mature depends upon being placed in a nurturing environment—i.e., the womb. In the end, however, the significance of the womb inexplicably falls away, with Donum Vitae simply announcing: “it is immoral to produce human embryos intended for exploitation as disposable biological material.” (DV, I, 5; also cited in the Catechism 2275).
For more read "Embryonic Confusion."