There's another variation on the "kill-a-life" argument that is important to refute. It goes something like this: "even if the aborted embryo is not yet a human life, it is well on its way towards becoming one; it is therefore wrong to terminate this potential human life". My fundamental problem with this argument is that equates potentials with realities, and there's no way this line of thinking is sustainable.

Imagine a healthy, fertile couple who choose not to have children. They could bear a potential human life, but yet they don't and no one will accuse them of "killing" that potential life (and even if they do have n children there's always the potential for n+1). Or think of a woman who plans on having only one child; by not aborting a first pregnancy, she is effectively closing the potential human life that would have arisen in a future pregnancy! Now, I could continue on and on with such examples, but I reckon the message is clear that talking of potentials will inevitably lead one into such existential quagmires. Moral of the story: keep realities away from potentials.

A slightly more sophisticated version of the "potentials" argument uses the moment of conception as keystone. In this view, the new and unique DNA combination that is obtained when the gametes fuse makes for a far more substantial "potential", and therefore one which should be preserved. Now, this is indeed a slightly better argument than the original one, but that does not make it valid. There's two main reasons why: one practical, and the other more conceptual.

On a practical level, the uniqueness of the new DNA combination doesn't mean much for mother nature. According to some estimates, up to 50% of pregnancies end in spontaneous abortions ('miscarriages') soon after conception. If those who cherish the new combination as sacred were to take their doctrine seriously, then for every person that is alive they should be mourning another that didn't make it. Oh the humanity! (Not to mention that current technology should allow for cold storage of this unique combination in case of a detected miscarriage — to be reused later again in a new pregnancy attempt. So, why aren't the supporters of this viewpoint clamouring for this preservation scheme to be put into practice?)

On a more conceptual level, the problem with this argument is that assumes the uniqueness of the new DNA combination as reason enough to justify its preservation. But this is a purely arbitrary choice: why not choose instead the also unique patterns (the structure of lungs, neural pathways, etc) generated during embryonary development and which are dependent not only on the genetic recipe but also on developmental factors? Moreover, bear in mind that DNA is essentially digital information (forget for a second the complications of epigenetic information — it can all be eventually reduced to the digital domain anyway), allowing us to put forth a thought experiment such as the following:

1. digitise the genome of the parents;
2. apply a virtual meiosis to each to produce two virtual gametes (use some stochastic algorithm for crossovers),
3. fuse the virtual gametes into a new unique DNA combination.

Now, redo steps b) and c) millions of times. Congratulations, you just generated millions of "potential" unique human beings on your computer! (Careful not to push that "off" button...)

Though not yet realistically possible, it won't be long before we have the technology to put the above experiment into practice. And time will come when it will even be possible to realise the crucial additional step of translating the generated DNA from a digital format into actual chromosomes ready to be implanted into an egg and then into an uterus. Do you see now the implications of assigning undue importance to the uniqueness of the new genetic combination and to potentials in general?