I don't have a perfectly clean, black-and-white answer for when the embryo becomes a person deserving legal protection. To me, that issue is largely shrouded in mystery, and I can accept that I may never have the kind of clear-cut answer that I or many others would like. I'd rather admit some uncertainty than set black-and-white limits that are wrong.
On the early end of life, I don't doubt that a unique biological being is created at the moment of conception. The DNA of the embryo at that moment will define an individual unlike any other. However, as I discussed in post 101 on this thread, most fertilized eggs, or embryos if you prefer, do not implant securely in the womb and do not survive their first menstrual cycle. If I believed that an embryo became a person at the moment of conception, then I would have to believe that over half of all souls died at less than a month of life and without ever having developed a heartbeat. I find this notion to be unbelievable, so I don't believe that an embryo becomes a person at the moment of conception.
At three weeks after conception, the embryo has developed to the point of having a heartbeat. At 40 days, there are measurable brain waves. It is only after that first few weeks that women normally realize that they are pregnant. Most women probably know before the brain waves start, but I doubt most women know before a heartbeat has started. I can respect people who've looked at the evidence and concluded that a heartbeat and/or brainwaves aren't enough to say that a person with rights exists. However, when I err to the side of life, this is where I land.
As a practical matter, I don't think it changes my position significantly. I would allow the birth control pill although many in the pro-life movement would ban it because some pills interfere with implantation. I would probably allow a true day-after pill, but I wouldn't allow RU-486. I would not ban in-vitro fertilization even though it produces embryos that will likely never be allowed to develop. I do not support embrionic stem-cell research because I do not want to push that envelope any further.
Two issues. First, I'm not sure why the large number of spontaneously aborted embryos has any impact on what embryos are. If you look at the vast expanse of human history, it wasn't uncommon for around half of children, once born, to die before their first year of life. Many societies turned a blind eye to, or even condoned, exposure and other forms of infanticide and even today, the practice is often easy enough to hide because so many infants normally die at birth or immediately afterward. But even then, people from those societies have expressed that infanticide was wrong and, like abortion, it was often practiced in secret and not discussed.
Secondly, many if not most of those spontaneously aborted embryos are so genetically damaged that they (A) could arguably fail the genetic test of being "human" and (B) lack any potential to ever mature beyond a short number of cell divisions so they have no future, which is the seems to be the key to granting personhood everywhere else (e.g., we deny personhood to the brain dead because they have no future recovery).
I don't normally deal with "souls" because no one can objectively show me when a person does or doesn't have one. But I wouldn't find it difficult to beleive that God can tell the difference between a fertilized egg with a future and one without them and if God is troubled by so many fertilized eggs never making it very far, He could easily know when to withhold a soul, just as He can tell the difference between a white blood cell and a baby or a human and an animal and give a soul to one and not the other.