Your Body, My Choice
Since choice is not always present with pregnancies, responsiblity is a relatively weak argument, and if responsiblity is defined as an obligation to another dependent individual, then we are back to the issue of whether the fetus has rights.
The way I see it, it is meaningless to argue as to the definition of an individual using biological terms because any definition can be accepted as valid depending on the feelings of the law-maker. Perhaps a type of brain-wave would be the criterion during one decade, and heart beats would become the de-facto standard the next. What makes one biological function more critical to the individual than the other?
Case in point: pro-abortionists claim that biological autonomy is the only criterion for individual rights and this is the current popular definition. Simply put, an individual can only be an individual if it is a distinct biological entity. Combine this with the globally held position that a woman's body is her own and therefore her rights over her body have priority over a non-individual, and you have a valid pro-abortion argument. Who's to say they're wrong? Just flip a coin and decide.
Let's take this reasoning to its logical, absurd conclusions:
For example, this argument would have difficulties once it encounters siamese twins that share organs. How would the legal system logically handle a case where the more dominant twin claimed the dependent twin was getting in the way of his rights over his body, and since this twin is not a distinct or autonomous biological entity, he has no rights and should be kill... I mean aborted?
Also note that rationally, a woman's right to her body does not have validity in abortion decisions unless the fetus has no rights of its own. To say that a woman's discomfort over getting pregnant and raising a child has priority over another individual's right to live is ludicrous. If this were the case, then in a world where Countess Bathory's physical needs were real, she would be allowed to kill virgins to alleviate her physical discomfort.
There is also the claim that a fetus takes over a woman's body or that it removes the woman's right to control her body. This may be a valid legal argument and some even claim it is legally equivalent to slavery since even a partial abrogation is an annulling of the right itself, but I don't think anyone would accept this argument if it involved murder.
So it all boils down to the argument of whether a fetus is an individual. If you maintain a religious viewpoint and believe in entities such as God and the soul, then there is no argument. If you believe that as soon as the sperm fertilizes the egg, an individual soul is created, then removing it would be murder. This is obvious.
But much less obvious and much more controversial is the fact that the globally accepted statement 'a woman's body is her property and therefore her right to control' (AKA the principle of self-ownership) is also not a valid argument according to some beliefs. According to these lifestyles, not only is your body not defined as being you, but it is also seen as a tool lent to you in order to perform certain duties. Do you have full rights over a hammer you borrowed from your neighbour?
Conclusion: The current definition of a fetus is illogical and inconsistent and abortion debates are pointless. If you are religious, most abortions are wrong. If you aren't then the best you can do is flip a coin or try to convince the law-makers that your definition is better. Murder is a matter of ad-hoc definition and the law-makers are the ones deciding whether you must keep the baby simply by defining what makes a human an individual with rights. The only relevant decision as far as controlling your own body is concerned is whether it's murder or not and this never was your decision to begin with. You have no choice and never did.