Abortion does not raise the risk of a woman suffering mental health problems, a major review by experts concludes.
Data from 44 studies showed women with an unwanted pregnancy have a higher incidence of mental health problems in general.
This is not affected by whether or not they have an abortion or give birth.
But anti-abortion campaigners said the review sought to "minimise" the psychological effect of terminating a pregnancy.
Experts from the National Collaborating Centre for Mental Health (NCCMH) used the same research methods they use to assess evidence on other mental health issues for NICE.
The work - funded by the Department of Health - came after concerns that abortion may adversely affect a woman's mental health.
Usually, a woman's risk of suffering common disorders such as anxiety or depression would be around 11-12%.
But the researchers said this rate was around three times higher in women with unwanted pregnancies.
The director of NCCMH, Prof Tim Kendall, said: "It could be that these women have a mental health problem before the pregnancy.
"On the other hand, it could be the unwanted pregnancy that's causing the problem.
"Or both explanations could be true. We can't be absolutely sure from the studies whether that's the case - but common sense would say it's quite likely to be both.
"The evidence shows though that whether these women have abortions - or go on to give birth - their risk of having mental health problems will not increase.
"They carry roughly equal risks.
"We believe this is the most comprehensive and detailed review of the mental health outcomes of abortion to date worldwide."
Prof Kendall said many previous studies had failed to adequately control for instances when women previously had mental health problems.
After a project which involved a three-month consultation, the researchers believe it would not "be fruitful" to carry out further studies into how pregnancies are resolved.
They say future work should concentrate on the mental health needs associated with an unwanted pregnancy.
Dr Roch Cantwell, a consultant perinatal psychiatrist who chaired the steering group, said the review was called for in 2008.
He said: "At that time, the Royal College of Psychiatrists issued a position statement saying the evidence on abortion and mental health was imperfect and conflicting.
"We all recognise abortion is a very sensitive and emotive topic. Our aim was not to debate the moral and ethical issues, but to focus on the available scientific evidence."
The scope of the review excluded reactions such as guilt, shame and regret - although these were considered important - and also assessments of mental state within 90 days of an abortion.
This was because the research was not about "transient reactions to a stressful event".
Sophie Corlett, director of external relations at the mental health charity Mind, said: "It is important that medical professionals are given the correct information to provide support for all women, but particularly those with a pre-existing history of mental health problems.
"This study makes it absolutely clear that this group is at the greatest risk of developing post-pregnancy mental health problems and should be given extra support in light of this."
Dr Kate Guthrie, speaking for RCOG, said: "Abortion, including aftercare, is an essential part of women's healthcare services, alongside access to contraception and family planning information."
And, in a statement the sexual health charities FPA and Brook said: "This review of evidence will reassure women who have had or are thinking about having an abortion that it's a safe procedure with no direct impact on their mental health."
However, a spokeswoman for the ProLife Alliance said: "Once again the politics of abortion blinds those who should be rigorously objective in assessing epidemiological evidence.
"This is a pick-and-mix report trying to minimise the psychological effects of termination of pregnancy in a way which does our so-called medical experts little credit."
And Dr Peter Saunders, chief executive of the Christian Medical Fellowship, said: "This new review shows that abortion does not improve mental health outcomes for women with unplanned pregnancies, despite 98% of the 200,000 abortions being carried out in this country each year on mental health grounds.
"This means that when doctors authorise abortions in order to protect a woman's mental health they are doing so on the basis of a false belief not supported by the medical evidence.
"In other words the vast majority of abortions in this country are technically illegal."
Public Health Minister Anne Milton said: "We are pleased to see the conclusions of this important review.
"The findings will be one of the many sources of information that we will use to inform our sexual health document that will be published next year.
"What is clear is that having an unwanted pregnancy has implications for people's mental health and wellbeing."