Before you read my tortuous logical discussion of why I think this statement is correct, let me make the statement. I believe that we must assume that life begins at conception because it is the most conservative approach. If it is the wrong approach, it does the least damage to the rights of humans. The worst case scenario for this assumption is not as bad as the worst case scenario for any other assumption.
This definition of "conservative" does not refer to the political spectrum, it refers to scientific calculation. If you give me a value for the radioactivity for a certain substance, it has a value for its error, e.g. +/- 20%. If you have ingested this substance, and I want to calculate how much damage the radioactivity will do to your insides, I will carry that error through the calculation. The total amount of radiation dose inside your body could be higher by a factor of 1.2, therefore I will tell you the number I have calculated times 1.2. This represents the worst case scenario, and is the most conservative estimate of the total radiation dose you absorbed.
When taking into account the consequences of the worst case scenarios for each assumption about when life begins (conception, birth, in between), the most conservative assumption is that life begins at conception. If abortion is allowed to continue, that represents a tacit agreement that life begins at birth (or at the very least, after conception). If, someday in the future, mankind has unassailable proof that life begins at conception, then every abortion carried out will represent a murder. The converse of this scenario, where abortion is stopped now and then we find out later that life begins at birth, is an infinitely more acceptable alternative to the first scenario.
The following is the essay I wrote first, last night, about this. I prefer the summary above, but I will not delete this because Randy has already commented on it.
The most polarizing issue in American politics. Pro-lifers argue that abortion is murder because life begins at conception. Pro-choice folk argue that a woman has a right to choose what to do with her body, and that it is not murder because we do not know when life begins.
Science can not yet tell us definitively when life begins. Even if science could tell us that, not everyone would be convinced (lets all think about how some "scientists" dispute that the earth is warming). So, given this lack of evidence, it is reasonable to make an assumption about when life begins. Should we assume that life begins at birth, at conception, or at some point in between? The consequences of each assumption are far reaching.
If we assume that life begins at birth, that would make abortion a perfectly allowable practice that government has no business regulating. Government, among other things, is charged with punishing the violator of another individual's negative rights, like the right not to be killed. If an infant is not alive until it is born, it can not be killed until after it is born.
If life begins at some period after conception, but before birth, then abortion is only a legal issue after that period has passed in a pregnancy. This assumption, of the three, appears to me to be the least defensible.
If life begins at conception, then, as pro-life activists say, abortion is murder. This means that the government must punish the mother who receives an abortion.
So, given these choices, which one should be chosen? The most important consideration is choosing which assumption causes the least amount of harm if it is chosen and its consequences carried out.
If life begins at birth, the mother has absolute power over the fate of her infant. This protects the mother in the event that the pregnancy was caused by something awful like rape, or that the mother experiences complications that risk her own life. Fortunately, cases like this represent a small percentage of the total number of pregnancies, and the total number of abortions. According to the abstract of this article, none of the most frequent reasons for abortion have anything to do with either of these unfortunate occurences (I cannot read the full article because I do not want to pay for it). Of course, women with complications rarely go to abortion clinics, so that is most likely under-reported. I must admit I do not know much about the motives for abortion. If someone can find a reputable source that says something to the contrary, I welcome the input.
If life begins at some point between birth and conception, then partial-birth abortions are murder, but first trimester abortions are not a legal issue. This assumption appears weak to me because if you grant that life starts before birth, then you must assume that life starts at conception in the absence of compelling scientific evidence confirming otherwise. Do you say that there is an arbitrary cut-off, like, say 18 weeks? Is the child alive when it kicks? When it starts to sprout limbs? To me, the strongest position that a pro-choice advocate can take is to unblinkingly accept that life begins at birth, that the baby is not alive and individual until it is outside its mother's womb. If you grant that life begins before birth, you have already lost. Every indicator, like kicking or a heartbeat, can be traced back further and further until eventually you have one cell dividing into two.
If life begins at conception, then the infant is protected and the mother is stuck. Unwanted pregnancy or not, she will have to go the full term. However, it seems apparent that the infant is totally defenseless, and that the mother is not. Unfortunately, in this case, an unexplainable miscarriage might be a legal issue as well as a personal tragedy.
The most conservative assumption is that life begins at conception. The least harm is done when one assumes that life begins at conception. The infant is protected in the event of an irresponsible mother. I think This assumption grants rights to an infant immediately. Whether or not this grants the unborn infant legal protection is a difficult question to answer and nearly impossible to legislate.