There is a cultural assumption here that makes the question somewhat complicated and difficult to answer without being anachronistic. While abortion is an ethically controversial topic in Christian culture, Aristotle lived over 300 years before the birth of Christ in a culture that did not particularly value the lives of infants, possibly due to a high infant mortality rate. It was considered perfectly acceptable to expose an unwanted baby on a hillside. Although the technology of safe abortions had not yet been invented, Aristotle's acceptance of infanticide would suggest that he would have found abortion completely acceptable in all circumstances.
The main argument in Aristotle's Nicomachean Ethics which would be relevant to this is his notion of humanity as defined by rationality. Our ethical obligations, according to Aristotle, are to rational beings. Thus a foetus or baby, lacking the ability to speak (a clear marker of rationality for Aristotle) or reason would not be considered fully human and we therefore would have no ethical obligations towards it. The ethical issue would simply be whether continuing to carry the foetus would contribute to the mother's well-being. In most cases, even in normal pregnancies, legal abortions by a qualified doctor are actually safer for the mother and have fewer complications than childbirth, and thus Aristotelian ethics would suggest that all abortions are ethically valid choices.