About 20,900 women get an abortion between 13 and 20 weeks, of which, if these results are respective of the UK, around 8,340 didn't realise they were pregnant.
BBC News
Late abortions reasons revealed
Many women who have late abortions had not realised they were pregnant, a study has found.
Southampton and Kent University experts asked 883 women why they had abortions in the second trimester - between 13 and 24 weeks into the pregnancy.
The researchers say women need more information so that they realise they are pregnant much earlier.
But anti-abortion campaigners said the rights of the unborn child should take precedence.
Around 11% of abortions take place after the 12th week of pregnancy. Just over 1% of those occur after a woman is 20 weeks pregnant.
Most abortions are permitted until the 24th week, although they can be carried out later if doctors believe the baby has a severe disability or if the mother's life is at risk
The time-limit for abortions is proving increasingly controversial, as medical advances mean even very premature babies can survive.
About 190,000 abortions take place in England and Wales annually.
Concerns
This study, which allowed women to give more than one reason for opting to have a late abortion, found women had not realised they were pregnant for two main reasons.
Just under 40% of women had not realised they were pregnant because they had irregular periods.
Another 31% had been using contraception, and so had not thought they could be pregnant.
And 41% said they had taken time to come to a decision about whether or not to have an abortion, delaying the procedure.
But 30% had simply not done anything once they suspected they were pregnant, with many waiting weeks to do a pregnancy test.
Half of those questioned were at least seven weeks pregnant when they were aware of their situation - a quarter were over 11 weeks pregnant.
Forty-two per cent said they had waited more than two weeks between requesting and having an abortion, and 23% waited more than three weeks, the maximum recommended by the Royal College of Obstetricians and Gynaecologists.
And many women said they had concerns about what was actually involved in an abortion, which had led to them waiting before going ahead.
Dr Ellie Lee, senior lecturer in social policy at the University of Kent, who worked on the study, said: "There has been a great deal of media and public debate recently about second-trimester abortions, especially those performed at 20 weeks and over.
"What has been lacking in this discussion so far is an understanding of why women have abortions at this stage.
"This means few have sought to properly address how women can be helped to terminate pregnancies earlier, and how policy makers and service providers might best think about the relevant issues for those women who seek second-trimester abortion."
She added: "If you were to reduce the upper time limit on abortion, it's arguable some women may feel rushed into a decision and may go ahead with an abortion when they would otherwise have had the baby."
Lack of knowledge
Ann Furedi, chief executive of British Pregnancy Advisory Service, which provides abortions, said: "'We frequently see women who did not know enough about their own bodies to recognise the early symptoms of pregnancy, especially if they have irregular or continuing periods, as many women do."
But a spokeswoman for the anti-abortion Alive and Kicking Campaign said: "The most significant finding in this study is that 41% of women indicated that the delay was due to indecision.
"We believe that the rights of the unborn child should take precedence over the uncertainty of the mother, an opinion that is shared by the vast majority of the public who are particularly concerned about late abortion."
Many women who have late abortions had not realised they were pregnant, a study has found.
Southampton and Kent University experts asked 883 women why they had abortions in the second trimester - between 13 and 24 weeks into the pregnancy.
The researchers say women need more information so that they realise they are pregnant much earlier.
But anti-abortion campaigners said the rights of the unborn child should take precedence.
Around 11% of abortions take place after the 12th week of pregnancy. Just over 1% of those occur after a woman is 20 weeks pregnant.
Most abortions are permitted until the 24th week, although they can be carried out later if doctors believe the baby has a severe disability or if the mother's life is at risk
The time-limit for abortions is proving increasingly controversial, as medical advances mean even very premature babies can survive.
About 190,000 abortions take place in England and Wales annually.
Concerns
This study, which allowed women to give more than one reason for opting to have a late abortion, found women had not realised they were pregnant for two main reasons.
Just under 40% of women had not realised they were pregnant because they had irregular periods.
Another 31% had been using contraception, and so had not thought they could be pregnant.
And 41% said they had taken time to come to a decision about whether or not to have an abortion, delaying the procedure.
But 30% had simply not done anything once they suspected they were pregnant, with many waiting weeks to do a pregnancy test.
Half of those questioned were at least seven weeks pregnant when they were aware of their situation - a quarter were over 11 weeks pregnant.
Forty-two per cent said they had waited more than two weeks between requesting and having an abortion, and 23% waited more than three weeks, the maximum recommended by the Royal College of Obstetricians and Gynaecologists.
And many women said they had concerns about what was actually involved in an abortion, which had led to them waiting before going ahead.
Dr Ellie Lee, senior lecturer in social policy at the University of Kent, who worked on the study, said: "There has been a great deal of media and public debate recently about second-trimester abortions, especially those performed at 20 weeks and over.
"What has been lacking in this discussion so far is an understanding of why women have abortions at this stage.
"This means few have sought to properly address how women can be helped to terminate pregnancies earlier, and how policy makers and service providers might best think about the relevant issues for those women who seek second-trimester abortion."
She added: "If you were to reduce the upper time limit on abortion, it's arguable some women may feel rushed into a decision and may go ahead with an abortion when they would otherwise have had the baby."
Lack of knowledge
Ann Furedi, chief executive of British Pregnancy Advisory Service, which provides abortions, said: "'We frequently see women who did not know enough about their own bodies to recognise the early symptoms of pregnancy, especially if they have irregular or continuing periods, as many women do."
But a spokeswoman for the anti-abortion Alive and Kicking Campaign said: "The most significant finding in this study is that 41% of women indicated that the delay was due to indecision.
"We believe that the rights of the unborn child should take precedence over the uncertainty of the mother, an opinion that is shared by the vast majority of the public who are particularly concerned about late abortion."
