The fetus-host (like that? 😊) relationship is unique; it is the only relationship in which the right to life of the one and the right to self-determination of the other cannot be reconciled. It is impossible to value both rights at the same time, should they conflict. In the case of a blood transfusion, need for a kidney, etc., there is always the possibility of intervention from an unexpected source; but, in the case of abortion, the host’s act of claiming her right to self-determination requires the death of the fetus. There is no possibility of intervention from an unexpected source. Occasionally, the fetus survives the attempt; you might call that an intervention, but it’s certainly not doing things the way nature intended.

Additionally, law in jurisdictions deriving their jurisprudence from common law has historically viewed failure to act differently from affirmative action. If I see a person drowning and am able to save him, the law does not punish me if I simply walk away. On the other hand, if I push his head underwater and hold it there until he drowns, I’ll be punished for murder. There are exceptions to this rule, e.g. doctors are required to act to save life; but, generally speaking, the law does not impose a duty to act, but merely proscribes bad action. The moral problem of the relative good involved in imposing a duty to act is one addressed by the law centuries ago, and it was decided that imposing a duty to act was worse than not doing so. Requiring people to give blood or sacrifice a kidney would be requiring them to act affirmatively, whereas proscribing abortion requires the host to abstain from acting affirmatively to take the life of what could be another human being.

So, although we don’t know if the fetus is a human being with rights worthy of protection, we must take the possibility into account in our analysis. Obviously, if it isn’t a human being, the host’s right of self-determination wins; but, if it is a human being, then you have one human being (the host) who wants to exercise her right of self-determination by murdering another. When the rights of two human beings are involved, their rights are equal; but rights exist in a natural hierarchy in which the right to life is supreme, inasmuch as it is the necessary precursor to every other right. Thus, if both host and fetus are human beings, the fetus’ right to life trumps the host’s right to self-determination.

Again, we don’t know (can’t prove objectively) that the fetus is, or is not, a human being; so, how does a society founded on the principles ours is chart its course? A society that values human life will say, “We don’t know that the fetus is a human being, but it reasonably could be; thus, we abstain from killing it in the interest of what would be a lesser right.” A society that does not value human life will say, “We don’t know that the fetus is a human being, and it reasonably could be; but we don’t care. The host’s right to self-determination is supreme.”

Obviously, the latter position is not logical, to say nothing of disrespectful of human life. You mentioned a connection between the banning of abortion and an increase in infanticide. I suggest another one: between allowing abortion and a ballooning of disrespect of human life in manifold ways, such as euthenasia, warehousing of undesirables … treating people as interchangeable and disposable. This is because all of these are expressions of the same underlying moral choice: the decision to devalue human life. Once you violate the principle in one venue, e.g. abortion, you have compromised its expression throughout your culture; and this cancer will eat its way through your society. I think we’re seeing this today.

Since it is impossible to have a final rational answer to the question of whether or not a fetus is a human being, we must decide how to treat it without that knowledge; and how we decide to treat the fetus will determine whether we continue as a society that generally values human life or as a society that does not.