Conception and Protection of the Result of Conception
CCC No. 2270 states:
"Human life must be respected and protected absolutely from the moment of conception. From the first moment of his existence, a human being must be recognized as having the rights of a person - among which is the inviolable right of every innocent being to life."
The relevant part of the Didache - the earliest known writing we have regarding Christianity's position on the unborn - states:
"2 The Second Commandment
2:1 The second commandment of the teaching is this:
2:2 . . . you shall not murder a child, whether it be born or unborn. Do not covet the things of your neighbor."
Citing the Didache and other Church documents, the CCC states even more explicitly in the relevant part:
No. 2271 . . . You shall not kill the embryo by abortion and shall not cause the newborn to perish . . . Life must be protected with the utmost care from the moment of conception . . ."
Finally, CCC No. 2274 states in relevant part:
"Since it must be treated from conception as a person, the embryo must be defended in its integrity, cared for, and healed, as far as possible, like any other human being."
Although not directly, the CCC comes as close as anything I have read in it as far as defining what constitutes conception:
"No. 2377 Techniques involving only the married couple (homologous artificial insemination and fertilization) are perhaps less reprehensible, yet remain morally unacceptable. They dissociate the sexual act from the procreative act. The act which brings the child into existence is no longer an act by which two persons give themselves to one another, but one that "entrusts the life and identity of the embryo into the power of doctors and biologists and establishes the domination of technology over the origin and destiny of the human person."
As I said, although it is not a direct "definition" per se, the above quote from the CCC seems to make clear that conception occurs when the sperm penetrates the egg, as that is the only "act which brings the child into existence . . . [by] the power of doctors and biologists . . ." in a petrie dish.
In an earlier post, I distinguished between a totipotent zygote and an embryo, but I do not believe the Church has ever made, nor will it ever make, such a distinction. The only way man can "distinguish" the two is by - as you suggest - separating certain "transitions" between the moment of the sperm penetrating the egg and the creation of additional cells, and even this can only be done through the use of an electron microscope.
From a purely medical/biological point of view, I have found the following definitions of "conception" (and even here, the line blurs on occasion):
1. a. Formation of a viable zygote by the union of the male sperm and female ovum; fertilization.
b. The entity formed by the union of the male sperm and female ovum; an embryo or zygote.
2. Fertilisation (also known as conception, fecundation and syngamy) is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo.
3. The process of becoming pregnant involving fertilization or implantation or both. (This definition, from Webster's, is the most inarticulate, as it does not really distinguish conception from fertilization and implantation, whereas the Church certainly does make such a distinction).
The Soul and its Creation
Teaching regarding the soul is a bit more vague.
The 8th General Council of Constantinople established the dogma that man possesses only one single spiritual soul: unam animam rationabilem et intellectualem habere hominem.
Synods at Constantinople and Braga both rejected the teaching that souls exist before their connection with a body.
Strict scriptural proof regarding when and how a soul is created does not exist. Nevertheless, many, many Church pronouncements through the years lead us to the "comfortable" theological position that, "each individual soul is created by God out of nothing at the moment of its unification with the body." Fundamentals of Catholic Dogma, Ludwig Ott, Sect. 15, No. 4
In his encyclical Humani generis, Pope Pius XII said, "The Catholic Faith obliges us to hold firmly that souls are immediately created by God."
"Modern Christian philosophy generally holds that the creation and infusion of the spiritual soul coincides with the moment of conception."Fundamentals of Catholic Dogma, Ludwig Ott, Sect. 15, No. 4
Ott cites for contrast of the above quote, a condemnation by decree from the Holy Office on March 4, 1679, of the following heresy: "1185 35. It seems probable that every foetus (as long as it is in the womb) lacks a rational soul and begins to have the same at the time that it is born; and consequently it will have to be said that no homicide is committed in any abortion."
Disingenuousness of Citing "Early Church Fathers" on when Life Begins
In closing, it is important to note that early theologians such as St. Augustine did not have the requisite knowledge of biology so as to permit them to comment with authority on matters such as when a child in the womb deserved protection.
The poster-person for how NOT to be a Catholic, Nancy Pelosi, raised this exact issue recently when she said, regarding abortion and the beginning of life:
"as an ardent, practicing Catholic, this is an issue that I have studied for a long time. And what I know is over the centuries, the doctors of the church have not been able to make that definition . . . St. Augustine said at three months. We don't know. The point is, is that it shouldn't have an impact on the woman's right to choose.”
Several cardinals and numerous bishops across the US immediately criticized Pelosi for not only misrepresenting Church teaching, but also because of her duplicity in citing "Church Fathers" on matters that they simply did not have the wherewithal to know at that time the information we have today.
This may sound uncharitable, but if it were possible for Pelosi to lose an election and NEVER be permitted to serve in a position where she could have a hand in deciding a matter on abortion, I would gladly serve extra time in Purgatory (assuming, God willing, I make it there).