I think reader Maria has cleared this up for us:

From The Soul of the Embryo: An enquiry into the status of the human embryo in the Christian tradition by David Albert Jones, London: Continuum Books, 2004; ISBN 0 8264 6296 0.

A great book but unfortunately difficult to get here in the States.

To quote relevant passages:

The first Christians followed the Jewish ethical principle that ‘we do not set aside one life for another.’ However, they faced a difficulty in accepting that ‘her life takes precedence over its life’, for they saw the life of the unborn child as equally inviolable. The practical question of what to do in a situation in which a woman’s life was threatened by her pregnancy was therefore extremely problematic for Christians. It is not altogether surprising that in the first thousand years of the Church’s history, theologians preferred to pass over this difficulty in silence and to speak of abortion in circumstances where they were clear that it was sinful. It was not until the late Middle Ages that Christian theologians begin to address directly the question of abortion to save the woman’s life.

One of the first to discuss this case was Antoninus of Florence. He declared that it was neither legitimate to kill the woman to save the child (by Caesarean section) nor to kill the infant to save the woman (by abortion). If the only way to save someone is by killing someone else, it is better to do nothing. However, he made one exception to this rule. Citing fellow Dominican John of Naples, he argued that before the soul was infused into the embryo (which, following Thomas Aquinas, he regarded as occurring at 40 days for males and 80 days for females) it was legitimate to abort the embryo to save the mother’s life. This was not homicide, strictly speaking. However, an act that destroyed the early embryo and so prevented a child from coming to be was very close to homicide, therefore it could only be justified to save the mother’s life. Furthermore, it it were doubtful whether or not the embryo possessed a human soul then it was not to be harmed. Antoninus only permitted abortion of the pre-ensouled embryo to save the mother’s life. Nevertheless, it was very significant in explicitly allowing an exception to the traditional prohibition. Antoninus had great authority and was followed by several theologians such as Sylvester Prierias (d. 1523) and Martin Aspilcueta (1493-1586), more commonly known as Doctor Navarrus. [pp. 178-179; emphasis added]

and

… a principle that was accepted as early as Antoninus of Florence: it there is uncertainty as to whether the soul has been infused, then it should be assumed for practical purposes that it has. It is unethical to risk homicide. [p. 188]

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From this, I would argue that it is up to Macguire and his ilk to point to the exact time when ensoulment takes place. In the absence of irrefutable proof, it is unethical to risk homicide.

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(And do see the first post below and attendant links for clarification of various issues, including the ensoulment issue and the weight of a theological opinion)

Update:

More from Mirror of Justice:

The book to consult on this issue is John Connery S. J.’s book "Abortion: The Development of the Roman Catholic Perspective" (Loyola University Press 1977). Father Connery explains that Antoninus relied on the distinction between the animated and unanimated fetus that was prevalent before the process of fetal development was understood. Antoninus condemned abortion of the animated fetus in all cases. For the unanimated fetus, Antoninus allowed abortion to save the life of the mother. According to Father Connery, this was completely dependent on the distinction between the animated and unanimated fetus and so I don’t think it is fair to cite Antoninus as supporting a pro-choice position. Later commentators who defend "abortion" to save the life of the mother (removal of a cancerous uterus) do so not because they defend the direct, intentional killing of a human life but because they regard the death of the fetus in such circumstances as incidental.

It is true that there have been individual Catholics who defend a "pro-choice" position (I suppose Daniel MaGuire, who has used the example of St. Antoninus, is one example). I don’t think this supports the view that the Magisterium has taken conflicting views on the moral permissibility of abortion. And, I don’t think it fair to use St. Antoninus’s views on abortion to support the view that the Church has taken different views on this question. From Father Connery’s account, St. Antoninus sounds more like Pope John Paul in Evangelium Vitae than Daniel MaGuire.