On the issue of abortion:
One reason that the right of a woman to terminate her pregnacy by abortion of the fetus was decided on the issue of privacy is that the government has no power to prescribe or proscribe medical procedures for a citizen. This is not necessarily bad law but it does hinge on when a fetus becomes a separate person whose rights are then protected by the constitution.
Therefore, the question of when does life begin does not apply so much as when does a fetus become a legally recognized person protected by the constitution? In other words, when is it no longer abortion but rather becomes infanticide? It is not at all clear nor need it be assumed that this is so at the moment of conception. The issue of abortion was not even on the legal radar screen until the early 20th century when issues that impacted women's sexuality became a matter of public concern. This was the era of the Comstock laws forbidding the dissemination of birth control information to women.
For most of history a child was considered a person at birth or sometime after birth. Some religious traditions still use birth as the demarcation for the beginning of legal personhood. Our technology allows us to know much more about the fetus in utero and this influences our feelings and ideas on the matter and that is one reason that this issue is of such importance in our society now.
As far as when life "really" begins--well that depends much on what you mean by "life". In a biological sense, reproduction is not so much the beginning of new life as it is the passing on of life either asexually (that is by cloning) or sexually (that is to genetically unique organisms). Certainly gametes (egg and sperm cells) are alive and yet they are not organisms. The same is true for somatic cells. Certainly, morulae (first two weeks from conception), blastulas (about 3 weeks from conception) , and gastrulas (about 4 weeks from conception) are alive and are organisms but they are in no way capable of independent life nor do they even remotely resemble mammals or human beings. A fetus (from about 12 weeks post conception) resembles a human being but is not yet capable of independent life and will not be for some time. And yet we are dealing with an organism that does have great potential to become an independent and unique being. This is where we are on the horns of the dilemma our society faces with the issue of abortion.
The dilemma is when do we limit the rights of a living person in order to accomodate the rights of a fetus? Does a fetus have rights?
Personally, I am not impressed by either pole of the controversy. I find the arguments of those who say that an abortion is no different than having a few cells or an organ removed ridiculous. We all know that these fetal cells are different and special and that difference had enormous ethical consequences. At the same time, I find the arguments or the extreme anti-abortionists equally ridiculous. They would have the life of a woman held hostage to that of a fetus and this does not make much biological sense. I would hate to see every spontanous abortion (miscarriage) investigated as a possible murder.
I have had two pregnacies in which I had preeclampsia--this condition has the potential to kill the mother and the fetus when it becomes severe. The only remedy is the delivery of the baby. I was fortunate in that my condition was moderate in the first pregnancy and in the second only became severe in the last month. Therefore, we were able to go to term with the first baby and induce at 36 weeks with the second (one month early). However, there are women who face emergency Ceasarians as early as 20 weeks in order to save their lives. This is before the fetus can survive outside the uterus. Should the government interfere with her decision?
I believe that these issues are best left to a woman, her family, her faith and her physicians because they have such an impact on her life--and that her right to life is protected.
These are not easy issues and the consequences of our decisions devolve onto real people who must make very real decisions. This is not a matter of abstraction only.