!!!!
The biological beginning (life) of a new human being/organism occurs in human sexual reproduction both in vivo, i.e. inside the body and in vitro, i.e. outside the body when a human sperm makes contact with the protective covering of and fuses with a human oocyte (before the “zygote” is developed). Examples include normal natural sexual intercourse, and artificial sexual reproduction in IVF/ART research laboratories and infertility clinics, although the latter sometimes does not involve a human sperm.
– Abyssum
ABYSSUS ABYSSUM INVOCAT / DEEP CALLS TO DEEP 
I think this is a splendid definition and one that is sorely needed. The Catholic pro-life movement absolutely must be grounded in the truth: scientific, philosophical and theological.
There is a very little known quotation of Saint Thomas Aquinas. It occurs in a highly theological context. Very, very few people know of this remark by the Angelic Doctor. And that is sad. St. Thomas says that the Ptolemaic theory of epicycles may “save the appearances” but that this is not sufficient proof that the theory is true, “for the appearances might perhaps also be saved on another hypothesis” [Summa Theologica Ia, 32, 1, ad 2]. In his Commentary on Aristotle’s De Caelo et mundo 92, c. 12, lectio 17) St. Thomas remarks that though a hypothesis offered in explanation of the movements of the planets seems to explain the facts, it does not necessarily follow that the explanation is true, “for the facts might be explained in another way as yet unknown to men.” Very wise sayings, I think.
I lived through the intellectual period when many Roman Catholic theologians were very quick to wed Christian philosophy and theology to Darwinian theories. The great German theologian Karl Rahner was quite quick to “throw in the towel” on many issues in the face of “scientific” truths. Many of his modern followers were quick to adopt his theories of hominization to “explain” how God acting through secondary causes brings about new life in line with evolutionary theory. I mention this because now there is a serious scientific movement within the biological sciences themselves which casts doubt on evolution. I cannot say how this will play out. But I think His Holiness, Pope Benedict is very wise to maintain a somewhat critical attitude toward science. Many accuse his Holiness of timidity. Many in the Catholic pro-life movement would like him to “do more” to advance the cause. I do not count myself as one of his critics. I think it is prudent for him to move slowly and carefully. Perhaps I am wrong. One wonders if those Thomists involved in the Galileo affair had been more true to the Angelic Doctor’s circumspection, whether things might now have turned out quite differently.
I am 100% pro-life. But at the risk of being a party pooper, I am not quite ready to hitch my wagon to everything that modern science says is so, even in embryology. Perhaps that is imprudent?