I would start out by making a personal observation. The two vocal positions on the subject (pro-life, pro-choice) when discussing the issue tend to respectively concentrate on the importance of life and the importance of choice. This is odd to me as it seems apparent that the real issue is the value that should be placed upon an unborn.
I shall give three possible decisions regarding an unborn's value and discuss them (obviously your viewpoint may be a combination of these or something else entirely):
1) There is some slow increase in the value of an unborn starting with none and increasing from then onwards until at some stage (say at birth) the entity is deserving of equal consideration to that of an adult.
There are points during the pregnancy at which the destruction of the unborn is equivalent morally speaking to killing an ant, an act of minor violence (e.g a slap), Grievous bodily harm, manslaughter and murder. The precise points at which these equivalences occur are meaningful legal questions but are no more difficult than deciding when a pile of sand has reached a series of prescribed weights.
2) The unborn is deserving of equal consideration from the moment of birth but before then has no special status.
According to this viewpoint it is entirely permissible to abort a foetus/embryo or blastocyste at any stage prior to birth. However, as soon as a new born emerges it becomes a heinous crime to destroy it.
3) The unborn is deserving of equal consideration from the moment of conception.
According to this viewpoint (if we are to take it literally) it is murder to abort a foetus/embryo or even a blastocyste of any stage of development. In particular a women may be committing murder by taking the morning after pill 2 days after conception.
So I shall deal with these viewpoints in order.
Firstly the slow increase in degree of consideration: This is the viewpoint I regard as being closest to the truth. I'd just like to put a couple of disclaimers down: The value of the unborn shouldn't be assumed to be some sort of currency that can be exchanged (two embryos for a foetus!) without significant changes to the moral questions; Also the increase in value may not be nice and steady for instance if the complexity of the mind and/or brain matters to you then the unborn gains most value towards the end of the pregnancy.
Secondly the position that birth is a magic time at which a new human life becomes something to value: I shall argue that viability is an ethical red herring. I shall also give reason for believing that a new born's mind has already started developing (probably in unique directions) before birth.
Viability: The options under consideration in this moral question are (a) have an abortion (b) do not have an abortion. Suppose we have two identical potential mothers Helen and Jessica living in identical universes. The only difference between Helen and Jessica is that Helen's unborn has a rare condition that has no effect on life after birth but makes it practically impossible for the unborn to survive should it be born at this stage. Jessica's unborn does not have this condition. Jessica and Helen are both deciding whether to have an abortion. There is no difference between the two scenarios in terms of the effect on any living creature of having or not having an abortion. Yet a morality that puts value on viability would make a strong distinction.
The mind: We know that the basics of walking, the ability to see, hear and touch are learnt in the womb. In the case of the eye we know that there are training patterns of activity on the retina that teach the brain to understand the concept of 2D space (I'd happily discuss these training patterns themselves for hours). There are good theoretical reasons to think that other things (including patterns of thought) are learnt in the womb. So the idea of a near term unborn as being entirely blank slate and hence, under some ideas of morality, of no value is really dubious.
Thirdly the position that conception is a magic time at which a new human life becomes something to value: I'm not going to look at the religious questions here (although I may be persuaded to come back to such questions later on). Apart from religious considerations it seems that the common reason given for this attitude is that the potential of the embryo is something that is valuable in and of itself. I am going to argue that this viewpoint leads to total moral absurdities.
Sometimes it is argued that although an individual blastocyste does not yet have anything remotely approaching individuality (in any meaningful sense) that its future uniqueness entitles it to be regarded as a person. I dispute this. Suppose my friend Helen and I are talking. By chaos theory my decision to utter the phrase "Shall we ask for the bill?" or not will result in two totally different universes in which Helen will end up having vastly different life histories (or indeed result in very different infants being born to her if she decides not to abort her unborn). Either decision will remove a unique future history of an individual from the realm of the possible. It is absurd to maintain that I am murdering someone whichever option I choose!
In which case we must surely maintain the uniqueness of an entity is measured as it now is. In which case if a women aborts her unborn at such an early stage that there is no real uniqueness of mind there (such as before 14 weeks when the nervous system starts to form) is she doing anything wrong? If this women aborts such an embryo and proceeds to have another child later we may as well view her to have mearly chosen to delay her pregnancy as far as morality is concerned. The later she leaves it and the more individuality the mind of the unborn has the more dubious this argument becomes. In any case we can only view her as destroying the complexity of mind that the unborn has already achieved.
So in conclusion I think the only sensible viewpoint is some version of (1). I have assumed a couple of points here including that there are no good religious reasons for aborting the child (I wrote this argument from a secular standpoint). I shall endeavour to come back to these issues when they come up in other discussions.