I am currently reading through a book called Who Killed Homer?: The Demise of Classical Education and the Recovery of Greek Wisdom by Victor Davis Hanson and John Heath. In the book they make the following observation: "We in the university have invented the very tenets of specialization. We have developed the strange notion that if we can find a single exception to a sound generalization then the entire thesis itself must therefore be rejected. Deeply suspicious of grand theories, we are schooled to be quibblers and clerks, to live in fear of having our work tainted with the humiliating label of 'popularization, of one scholar finding one exception to a sensible principle of history or literature" (Encounter Books, New York, NY, p. 24). The entire book examines Greek thought and the detriments to our culture that rejecting Greek wisdom has wrought. This passage struck me as particularly poignant. In no other discussion do I see the rejection of Greek thought (such as the reality of human nature) more than in my discussions on abortion.
The question of when human life begins has already been settled. Embryologists consistently agree that human life begins at fertilization. This is even accepted by pro-choice academic philosophers. But desperate to try and prove that there is nothing immoral about abortion, the average pro-choice person will try and argue that the unborn are not actual human beings by pointing to exceptions. Because teratomas and hydatidiform moles sometimes result from the sperm and egg fusion, this means that a human being never results from the sperm and egg fusion and begins sometime later. Embryologists (who are the experts) surely know about teratomas, hydatidiform moles, that human embryos can twin and fuse, etc., yet they still consistently agree that human life begins at fertilization. If pro-choice and pro-life embryologists agree on this point, we really should not be arguing it.
It is a true fact that sometimes non-human entities result from the sperm and egg fusion, but it doesn't follow from this that a human being never results from it, nor does it follow (as I have sometimes heard) that it begins as a human organism then "becomes" a teratoma. The reality is whatever it is it was ontologically from the beginning. No teratoma or hydatidiform mole begins as a human being then changes into a teratoma or hydatidiform mole. See this linked article for a more in-depth treatment of these issues.
Then sometimes they'll appeal to the fetus-in-fetu, which is a bizarre and horrifying situation in which one twin absorbs another, and essentially becomes "pregnant" with what remains of the original twin. As pro-life advocates, we don't claim that there is no mystery when it comes to the process of human development. But appealing to bizarre cases such as the fetus-in-fetu, like appealing to non-human entities that result from the sperm and the egg, does not disprove the sound theory that human life begins at fertilization. The fetus that was absorbed may still be a human person, but it is no longer alive and so there is nothing immoral in having it removed.
You can't respond to a sound theory by appealing to exceptions or hard cases. The science is quite secure in that human life begins at fertilization. These cases might be tragic but in no sense do they present a challenge to the reality of human development.