In Islam we have it as a matter of doctrine that the soul does not enter the fetus until 120 days, so abortions before this, though not indulged in for petty reasons, are not considered to be "murder" as they would to a modern day conservative Christian.
There's also the other point, that we don't automatically rule on the value of the fetus' life over the mother's, even if we're talking about a possible late-term abortion. It's justifiable to save the mother's life if it is threatened. A jurist could make such a ruling.
What I wanted to point out was in regards to the first point.
Some scholars have concluded that early Christians took a nuanced stance on what is now called abortion, and that at different and in separate places early Christians have taken different stances. Other scholars have concluded that early Christians considered abortion a sin at all stages; though there is disagreement over their thoughts on what type of sin it was and how grave a sin it was held to be, it was seen as at least as grave as sexual immorality. Some early Christians believed that the embryo did not have a soul from conception, and consequently opinion was divided as to whether early abortion was murder or ethically equivalent to murder.Christianity did not have a doctrinal position on the exact time of "ensoulment", so earlier Christian scholars inherited philosophy on the matter from Aristotle and others. This was a point on which the debate pivoted for quite some time until the 19th century when the Catholic Church decided to default it to conception.
Augustine affirmed Aristotle's concepts of ensoulment occurring some time after conception, after which point abortion was to be considered homicide, while still maintaining the condemnation of abortion at any time from conception onward. Aquinas reiterated Aristotle's views of successive souls: vegetative, animal, and rational. This would be the Catholic Church's position until 1869, when the limitation of automatic excommunication to abortion of a formed fetus was removed, a change that has been interpreted as an implicit declaration that conception was the moment of ensoulment. Consequently, in the Middle Ages, a less severe penance was imposed for the sin of abortion "before [the foetus] has life".
The theological issues aside (What was the justification? Were the earlier Popes wrong?), there is a metaphysical point to be made. The idea that life begins at conception is a very materialistic one (in the sense of metaphysical materialism). As Western culture moved to such materialism (and also away from Judeo-Christian tradition), the Catholic Church seems to have moved right along with everyone else. Christians, who should believe in a soul, now don't even bring it up at all in the debate. All they are concerned with is the physical, the material: the zygote. Their blind adherence to this doctrine (which, from all indications, seems arbitrarily concocted by the Catholic Church in the 19th century) is a manifestation of this extreme materialism. Perhaps we can give the Catholics a pass, because they must follow the Church without question. But the Protestants have no excuse, their reasoning is purely materialistic (unsurprising in the context of their other positions on social issues which represent Ayn Rand more than Jesus Christ).
Just one of the consequences of the old clashing with the modern shift in philosophical outlook. Though metaphysical materialism is certainly an extreme by religious standards, it has had its upside (the increased focus on the worldly life makes people work to make it better... but when done in this way it comes at the expense of concern for the next life).
Regarding the materialistic answer to the question of when exactly can we consider a fetus to be alive
It's also not so simple to call a zygote not "alive", because people will take issue with that definition of "alive". If left alone in a womb, it will continue growing. Cells can be alive, they can die. We certainly know what a dead cell looks and behaves like, so it stands to reason the opposite is life.
I think the materialistic debate really comes down to a question beyond materialism: utilitarian ethics. Yes, an embryo/fetus, when left alone, can grow into a person, can be born, can grow up, can contribute much to society and their own fulfillment. But those are a lot of "cans". The mother is in the here and now, she's already a living, breathing adult with all the attributes of sentience, plus a safe and early abortion (that can be facilitated with the law on her side) allows her to have another child (or more) soon again whereas the risks with the fetus are a lot more. The life of the mother is clearly more valuable to society under the typical circumstances.
In a way, I find the debate kind of ironic because it's kind of a flip of the evolutionary origin of man debate. Darwinian evolution proponents ("scientism", not necessarily atheists, scientists, or anyone who already believes in the evolutionary origin but those who put such an emphasis on it so as to behave downright theologically in their treatment of evolution... cue: evolutionary psychology and other similar pseudoscience) will adhere to a materialism similar to the sort that the Christian conservatives strongly hold to on the issue of life beginning at conception. On the other hand, the arguments expressing skepticism (not necessarily opposition) of the meaningfulness or relevance of evolution to the issue of man's origins tend to take a similar route through ontology as in the objections to the "life begins at conception" position (regarding what it means to be a person and how to distinguish that while limiting arbitrariness).