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.
The majority of Christians believe life begins at conception, which means abortion is comparable to murder.
- "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. 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. 
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
Abortion is allowed but not to be undertaken casually. If the mother's life is in danger, abortion is mandatory. 
- 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