Abortion is the medical practice of terminating a viable pregnancy. There is disagreement as to when a pregnancy begins: either at when sperm fertilizes the egg or the implantation of the egg into the lining of the uterus.  In modern times abortion is a surgical procedure or is medically induced 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. Both pregnancy and abortion carry a variety of medical risks.
Medically speaking, an abortion can be spontaneous (miscarriage) or therapeutic (induced). This article treats abortion as synonymous with induced abortion.
People who believe that elective abortion should be legal as called "Pro-Choice" - choice in this case referring to the woman's 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 access to abortion should be legally limited, or that it should be illegal, are described as "Pro-Life." Religious belief and a pro-life attitude typically go together , but one by no means implies the other. The terms "pro-choice" and "pro-life" are examples of political framing.
Abortion is allowed by law in many Western nations, including nominally Christian nations such as Canada and the United Kingdom (except Northern Ireland). It is more regulated in Ireland, Poland, and most of South America and Africa. In many of these jurisdictions, it is often allowed if the pregnancy endangers the woman's life.
An ectopic pregnancy occurs when the egg implants outside of the womb, typically in a fallopian tube. In the UK, it occurs in 1-2% pregnancies.  The fetus will almost never survive an ectopic pregnancy and the pregnant woman's life is in grave danger without medical intervention i.e. ending the pregnancy (an abortion, loosely speaking). For this reason, ending an ectopic pregnancy which saves a woman's life is allowed in almost every country.
Medical practitioners define abortion as only applying to "intrauterine pregnancies" or "otherwise viable pregnancies", which would exclude ectopic pregnancy.  Pro-life campaigners agree that ending an ectopic pregnancy is not an abortion.  While medically speaking, ending an ectopic pregnancy is not abortion, according to pro-life definitions it is murder. Theists may defend allowing this procedure by referring to the "principle of the double effect", i.e. murder is not the "intent" when saving the woman's life.
- "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.
- "The pro-life movement supports the exact policies that will keep abortion rates high. It is those who believe in choice who support policies that will bring the abortion rates down. "
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 spontaneously 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:
- Life means it can survive independently, which the zygote/embryo cannot.
- Life starts at a different stage, such as implantation, beating heart, or at a certain time.
- Various scriptures seem to imply that life starts at a different stage.
Principle of the double effect
The Catholic church does not allow "direct" abortions but does when it is an "unintended" result of some other essential medical procedure.
- "The mother is not saved by the death of the child but by the removal of the tube. Because the death of the child in this case is a side effect which is not intended, and because the saving of the mother's life is not brought about by the death of the child, such a removal of the damaged portion of the tube is morally permissible. The ethical rule that applies here is called the Principle of the Double Effect. "
This arguable places too much emphasis on intent and essentially ignores likely outcomes of an action which is the usual basis of any decision. It also prevents early intervention which may be less medically risky for the pregnant woman. Insisting that the fallopian tube be removed in cases of ectopic pregnancy seem unnecessarily cruel and reduces her fertility.
- "In the event of an ectoic pregnancy, you may remove the malfunctioning tube, which unfortunately also will cause the death of the baby. Other techniques, such as methotrexate or microsurgery to remove the pregnancy while leaving the tube intact are immoral. "
- "So a woman can have her whole tube removed (an unnecessary procedure that could reduce her future fertility), but she can not have the pregnancy plucked out (as is done with the standard therapy, a salpingostomy, where a small incision is made in the tube and the pregnancy removed) and she most certainly could not have the methotrexate. "
The pro-life movement is often associated with religion or Christian belief. However, a minority of atheists are pro-life.  Some of the apologetics used is secular in nature.
Alleged negative health impact
- "Emotional and/or psychological problems can arise when women deny or suppress the natural grief that results from the loss of their aborted child. [...] Abortion can also have physical effects on women and more and more evidence is arising as to the link between abortion and breast cancer. "
Not handling grief in an appropriate way is a separate issue from the harm of abortion. However, various studies are beginning to conclude there is no increased mental health risks for first trimester abortions. Research is continuing for late term abortions. Carrying an unwanted child to term also has psychological costs.
The consensus of all mainstream scientific bodies is that abortions are not associated with a higher incidence of breast cancer.
Pro-life websites continue to spread misinformation that is contrary to the mainstream medical consensus.
Pro-life movement seems to want to legislate morality. They argue that since murder is illegal, we can also make abortion illegal in most cases.  Attempting to legislate morality raises the question: whose morality should we use? Also, there are moral principles that probably should not be enshrined in law, such as adultery or religious dietary laws.
Contrary to the rights of the father
- "Such a "choice" also imposes on the father by depriving him of fatherhood and the right to protect his own baby"
Pro-life movement opposes contraception
The pro-life movement generally opposes the use of contraception.  Since contraception an effective method to reduce the abortion rate and banning abortion is ineffective, it shows the movement is not really about saving unborn lives but about social control, particularly of women.
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. 
While atheists tend to be pro-choice, there is a significant minority that are pro-life. A 2012 Gallup poll  of US adults found 68% of non-religious identified as pro-choice and 19% identify as pro-life.
A significant number of pro-choice atheists believe that abortions should be minimised, and either pro-choice is the way to achieve it or believe the pregnant woman should have the final say.
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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
- George Monbiot, Who’s driving high abortion rates? It’s the religious right, 13 Jan 2016
- Pro-life Humanists