The question of when human "life" begins is a matter of some controversy. Part of the problem in this debate is a failure to define terms. "Life," for example, is too vague a word. A human has "life," but so does a kernel of corn. Since the issue is often phrased in terms of "murder," and since no one would consider it "murder" to end the "life" of a kernel of corn, we will need to come up with a more focused term to express what we mean.
For this first graph (on right), I have chosen the term "humanity," although any term will do as long as we can agree on what it means. This graph represents a starting place - something upon which, I hope, most of us can agree. Although an unfertilized human egg or sperm cell, may have "life" in some sense, most of us are agreed that "human life" (in the critical sense) does not begin before conception. This is represented by the blue line at the lower left of the graph. The quantity of "humanity" present before conception is zero. There is nothing human (again, in the critical sense) to kill.
Likewise, most of us will agree that, after birth, a child has full claim to human life. This is represented by the blue line at the upper right of the graph. After birth, most of us are agreed that the child is one hundred percent human - but, in any case, we must all be agreed that killing it, at this point, would be one-hundred-percent murder. The problem is what to do with the area in between.
There is an old joke which goes, "There are two kinds of people: those who divide all people into two distinct groups, and those who don't." Here, I will attempt to sort all people into two warring camps - with the understanding that this simple division will not be completely realistic.
The Pro-Life view (graph on left) assumes that the quantity of "humanity" takes a one-hundred-percent quantum leap at the instant of conception. It asserts that a fertilized human egg cell has as much claim on humanity as a full-term unborn child - that something "mystical" happens as the sperm passes through the membrane of the egg. This view might strike a strict materialist (those denying all spiritual elements) as being highly unlikely. However, most of those who take this position seem to believe in some form of "mystical" basis for the human soul or spirit.
In with this group, I will include those who believe that birth control is a "sin." Although most in this group will not necessarily consider this sin to be equivalent to murder, we must allow for a gray area here; some might place "humanity" to the left of conception.
Likewise, the Pro-Choice view (graph on right) assumes that the quantity of "humanity" takes a similar one-hundred-percent quantum leap - but at the instant of birth. It asserts that a full-term unborn child has no more claim on humanity than a fertilized egg cell does - that something "mystical" happens as the child passes through the birth canal. This view might strike a strict materialist as being highly unlikely. However (and this is truly surprising), most of those who take this position seem to hold somewhat materialistic views - in fact, many of them regard all "mystical" beliefs to be "hogwash."
In with this group, I will also lump all of those who place their one-hundred-percent quantum leap somewhere between conception and birth. These people might argue that a woman has a "right" to an abortion any time before, perhaps, the second or third trimester.
There are yet others who would argue that a parent should have the right to an "abortion" for some time period following birth. Some have suggested (not always completely in jest) that this period should include the teenage years. Although this suggestion might sound horrifying, it can at least be argued that this would be more of a death sentence for unacceptable behavior than an arbitrary murder of a completely innocent child. If a court of law were involved, "due process" could even be served.
Of course there are always other considerations such as rape, incest, or endangerment to the mother's life; different people will take different positions regarding each of these cases. But what I find to be very interesting is that virtually all people seem agreed that there exists some kind of "mystical" quality (like a soul) which either is or is not present at any given time - and which does not seem able to assume any value in between.
If "materialism" is to honor the scientific reality it proffers as it's foundation - and not be rightly regarded by the rest of us as some kind of foolish "mystical anti-mysticism" - then it's supporters will need to revise their position. By now fetal development is sufficiently well understood that (one would think) the proposed "mystical" quantum leap (apparently implicitly agreed upon by all camps) would have been abandoned by the materialists, at least. The physical reality is explained, in copious detail, many places including this link here.
The developing child grows, gradually and constantly (as depicted in the graph below) from a single cell to full term. This process requires some 38-40 weeks and is called the gestation period. The child grows about half an inch each week (in very rough terms), adding various new bodily functions as development proceeds. There are many small quantum leaps along the way (for example, the heart starts beating by about the 4th or 5th week and neurons in the brain start firing by the 20th or 21st week, etc.) but the overall picture is a gradual process. After about 25-28 weeks the child is sufficiently developed that it might even survive outside the womb (with sufficient intervention). By 36 weeks, its survival chances are good. By 38 weeks, the child is considered to be full term.
This graph (now left) reflects what ought to be the materialist's scientific understanding of fetal development; the transition from zero to one-hundred-percent humanity must be a gradual one. This graph also suggests what the materialist's view ought to be regarding the proper legal consequence of abortion. Whether an abortion would constitute merely "a woman's choice" or would, in fact, be "murder" would depend on where along the development curve the aborted child was. At the bottom left, a woman's choice might be the driving consideration - but higher up, it is clear (even under a purely materialistic view) that other considerations must become increasingly weighty. Half way up should correspond to about "half of a murder."
Now the concept of "half a murder" might sound silly at first exposure, but society has to deal with similar kinds of gray areas all of the time. The penalties for multiple murders, for example, are greater than those for single murders. We have come up with reasonable laws to deal responsibly with many different degrees of crime. Other circumstances such as "intent," "self-defense," "due process" and "accomplice" are also given the necessary consideration. Crimes such as dropping large rocks off of freeway overpasses, firing a gun into the air in a crowded city, crimes of vengeance, or merely crippling an intended murder victim, can all be dealt with in a reasonable manner. We may disagree in some cases with the exact penalties which society has selected, but we can at least understand the principle and the reasoning behind such laws.
Using this same graph to get a very rough, first-order approximation, we might conclude that an abortion in the middle of the seventh week (one sixth of the full term) might be assigned the same penalty as a crime which (intentionally) had about a seventeen percent (one sixth again) probability of killing a person. Probability, although quite a different concept from a fraction, is expected to have the same general effect over time: Assuming the same levels of "intent" and other critical circumstances, six abortions of "one sixth of a human" might be assumed to have about the same expected overall consequence as would six "games" of Russian roulette with a newborn child. In any case, it is obvious (even assuming the materialistic view) that the crime can be far more serious than is assumed by those who speak in terms of "a woman's choice." Near to one extreme, using an Intra Uterine Device (IUD on the graph) might be assigned a penalty which is not particularly significant (at least as plotted on the next graph below). At the other extreme, a partial birth abortion (PBA on the graph) would probably be regarded to be indistinguishable from the murder of a newborn child. (In the difficult case of rape, society might decide to hold the rapist accountable for the penalty - assuming his victim chooses to abort her child.)
The argument can certainly be made that a straight line is not exactly the correct shape for the development curve. A straight line reflects fetal length, but others might consider its surface area (which varies as the square of length) or even its weight (cube of length) to be more appropriate guides. A possible improvement over a straight line has been plotted here (right). This more closely reflects the developing child's area/weight (instead of its length) as a gauge for its "humanity" - thus flattening the bottom end of the curve. It also takes into account the fact that the child is considered "full term" (or nearly so) for some time prior to birth - flattening out the top of the curve as well. This "smoothed out" curve may be closer to the truth; unfortunately, it also complicates the math somewhat. (The improved mathematical function might be something like: 100%_humanity * SIN2(pi/2 * time_since_conception / gestation_period) .)
All of this would need to be considered by those legislators who might eventually have to make the laws, and they would certainly also be encouraged to consider the advice of scientists, doctors, theologians, various organizations, and any voters who believed they had a stake in this matter. I doubt that either side will be willing to abandon their convictions in favor of a "middle" position such as has been described here, but it is at least obvious that the pro-"choice" advocates have staked out a logically-inconsistent position (at least from a materialistic point of view). Pro-"choice" advocates might do well to bring their position closer to this middle position, to eliminate the supernatural-styled quantum leap from their arguments. Pro-"life" advocates might also do well to support this position, since it focuses on the inconsistency in their opposition's position, instead of presenting their own position as a religious exception. They might also do well to take note that this middle position would still carry penalties which would be severe enough to cause some major changes in how society handles abortions.