Abortion is the medical practice of terminating a pregnancy. Abortion is allowed by law in many Western nations, including nominally Christian nations such as Canada and the United Kingdom.
People who believe that abortion is acceptable are described as "Pro-Choice" - choice in this case referring to the right to choose whether or not to have an abortion. Availability of legal abortions has societal benefits, individual benefits and prevents risky unregulated abortions.  Those who believe that abortion is unacceptable are described as "Pro-Life." Religious belief and a pro-life attitude commonly go together, but one by no means implies the other. The terms "pro-choice" and "pro-life" are examples of political framing.
In modern times abortion is a surgical procedure or medically included using drugs, but the ancients had their forms of abortion as well. In ancient Greece, for example, the practice was to have the pregnant woman drink a poison that would force her to have a miscarriage.
- "Since the first century the Church has affirmed the moral evil of every procured abortion. This teaching has not changed and remains unchangeable. Direct abortion, that is to say, abortion willed either as an end or a means, is gravely contrary to the moral law "
This teaching applies even in cases where the pregnancy was caused by rape. A significant number of Christians have abortions, despite this teaching.
- "More times than I can count, I’ve heard this remorseful comment from a post-abortive parent: 'I knew it was wrong to abort my child. I was brought up in the church and was pro-life. But I did it anyway.' "
In fact, the abortion rate of Christians and non-Christians in the US is about the same or even higher for believers! (Although abortion rates vary based on the specific denomination and other factors).   This may be largely due to the lack of sex education and contraception usage in more religious groups. If we consider abortion as bad, this is an example of social harm caused by religion. The behaviour of Christians in this regard is evidence that, for many, their faith is not as sincere as they tell themselves.
In 2012, Irish anti-abortion laws prevented Savita Halappanavar from receiving an abortion which could have saved her life.  The law was religiously motivated.
If abortion was wrong, it is strange that it is not explicitly addressed in the Bible. Sometimes James 2:26 is cited  but this only says the spirit must inhabit the body to consider it alive, not that life begins at conception. The current Christian teaching is based on questionable interpretation of certain verses. Murder and causing a miscarriage do not carry the same penalty in the Old Testament, therefore they are distinct Exodus 21:22-23 . The Bible contains several other verses that contradict the Christian view. 
Banning abortion does not prevent abortion
While many religious and non-religious people agree that abortion should be minimized were possible, disagreement often arises as to how this should be achieved. The approach of banning abortion by law is ineffective because banning abortion does not significantly reduce the rate of abortions.  This is possibly because women seeking an abortion procure one regardless of its legality.
All that an abortion ban achieves is increasing risk to the health of women.
Steps that have been shown to reduce the abortion rate are resisted by some Christians on religious grounds, such as increasing availability to contraception and improving sex education. Since fundamentalist Christians prefer ineffective methods for reducing the number of abortions, they are hypocrites when they call for less abortions.
Calendar-based contraceptive methods are murder
The some denominations, such as the Catholic church, consider calendar-based methods, which includes the rhythm method and NFP, to be an acceptable form of contraception. It is the only form of contraception if church teachings are followed and is often practised despite its unreliability.
- "The Catholic Church supports the methods of Natural Family Planning (NFP) because they respect God's design for married love. "
The rhythm method produces fertilized embryos that are incapable of surviving because the uterus is not in a state that makes embryo implanting likely.  This causes a high likelyhood that the embryo will be aborted as part of the menstrual cycle. The likelihood of embryo death is higher with calendar methods than some other contraceptive methods. By their own logic, this makes the church's support of calendar-based contraception the same as calling for the murder of embryos!
- "Some proponents of the pro-life movement argue against morning after pills, IUDs, and contraceptive pills on grounds of a concern for causing embryonic death. What has gone unnoticed, however, is that the pro-life line of argumentation can be extended to the rhythm method of contraception as well. Given certain plausible empirical assumptions, the rhythm method may well be responsible for a much higher number of embryonic deaths than some other contraceptive techniques. "
Ultimately, this illustrates that "pro-life" Christians are not really concerned with embryo death but use this reasoning as an ad hoc justification for their arbitrary beliefs about abortion and contraception.
Life does not begin at conception
Critics of the argument that life begins at conception point out that:
- Biologically speaking, life is "A distinctive characteristic of a living organism from dead organism or non-living thing, as specifically distinguished by the capacity to grow, metabolize, respond (to stimuli), adapt, and reproduce" 
- Sperm and ovum are alive before conception.
- Although sperm and the ovum cannot survive for long outside the body, neither can the zygote.
- Although it seems arbitrary to define life beginning in terms of stages of pregnancy, such as when a heart beat is detectable, it is just as arbitrary as choosing conception as the beginning of life.
- The Bible implies that the first breathe is the beginning of life, not the physical assembly of the body. Genesis 2:7 
- The Bible does not equate killing a foetus with killing a post-birth human. Causing a miscarriage does not carry the same penalty as murder. Exodus 21:22–24 Lev. 24:17
- Contrary to claims of Christians,  the Bible does not state that life begins at conception. These Christian teachings are based on a very loose interpretation of poetic language in the Bible. Some verses imply that a person "exists" before they were conceived, which is absurd, but this still does not necessarily imply the soul has entered the body at conception.
- If a zygote is a person at conception, a zygote could later split to form twins. If a zygote is a person, then twins are in fact the same person! Reductio ad absurdum.
- Up to the late 1960's, most evangelical Christian leaders were pro-choice!  Their recent attempts to prevent all abortion is a modern innovation.
- The words "life" and "person" are subjective labels that humans subjectively apply to situations and do not provide any solid basis for morality. The whole debate on when life begins is therefore moot.
Abortion is allowed but not to be undertaken casually. If the mother's life is in danger, abortion is mandatory. 
The Qur'an does not explicitly mention abortion.  This seems a strange oversight. Beliefs within Islam vary with some believing it is permissible and others saying it is forbidden. Muslims generally accept that an abortion is allowed if continuing the pregnancy would endanger the mother. Abortion is generally more acceptable in the earlier stages, with different schools of jurisprudence specifying abortion is allowed before a certain number of weeks of pregnancy. There are differing views as to when the soul enters the foetus, ranging from life begins at conception to 120 days. 
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- The Abortion Debate on ProCon.org
- "Abortion: Is it Possible to be both 'Pro-life' and 'Pro-Choice'?" - an essay by Carl Sagan and Ann Druyan