Almost always, opponents of abortion object on moral, rather than pragmatic grounds.
That is to say, objectors to abortion do not cite possible societal issues like increasing crime, deteriorating public health or civil unrest as reasons against abortion.
Instead, they take a moral position that abortion is murder. On account of this, and for the sake of brevity, I will set aside the few social concerns that might be raised, noting that social conditions are the impetus for abortion, and not the reverse.
As a corollary to equivocating abortion with murder, objectors must believe that a foetus is morally equivalent to an adult human. Whether the objection is religious or secular, without that assumption any moral argument would fall short. As a result, proving the assumption of moral equivalence false immediately defeats a moral objection to abortion.
This assumption takes two different, yet equally invalid forms: the first is secular and the second is spiritual. In the former, objectors assert that a fetus is biologically equivalent to an adult human.
The objector will note that the fetus’s DNA is human, that the fetus’s body is human-like, that the brain of a fetus shows signs of activity or some combination thereof. This reasoning falls flat since there is more to being human than DNA and appearance. The most compelling point made by the secularist is brain activity, and even that is weak.
The brain activity of a fetus is not comparable to that of a human’s until the beginning of the third trimester, which is the most commonly accepted cut-off for abortions. Prior to this, any biological similarity between the born and unborn is only skin deep.
The spiritual objection is simpler and more compelling, citing the ensoulment of the fetus as the source of moral equivalence. The heart of the objection is often an intuition of the spiritually inclined that something which will eventually have a soul should always have a soul, and thus even the recently conceived have them.
Though appealing for its simplicity, this intuition is ultimately quite faulty. It seems pretty clear that a soul cannot exist in the material world without a body and, more importantly, without a human mind.
A recently fertilized egg has neither of these traits, nor does a young fetus. Indeed, as I mentioned, a fetus doesn’t develop a human-like mind until the beginning of the third trimester.
How can a fetus which doesn’t even thinks be the seat of such a complex thing as a human soul? Similarly, why should a soul be spontaneously generated when the egg is fertilized?
A more reasonable belief would be that as the fetus develops into a full human, it becomes ensouled gradually. Once the fetus reaches the third trimester and begins to think in a human-like way, it might be able to be considered ensouled or partially ensouled.
Prior to advanced cognition, though, the idea that a fetus could be ensouled is ridiculous. Unthinking foetuses have no spark of cognition, no special recognition of themselves. They react to external stimuli mechanically and without thought, and even that only develops after eight to ten weeks.
What purpose does a soul have with an embryo or unthinking fetus? None.
Where does the argument against abortion stand, then?
The secular biological analogy between fetuses and adults only comes together at the onset of the third trimester, and the idea of an ensouled foetus is ridiculous until the same time period.
At the end of the day, advanced cognition is a requirement for being considered human, and how can something have a human soul if it isn’t human?