Most women who have had late abortions say that they had not known they were pregnant for up to three months.

An extensive study of women who had abortions between 13 and 24 weeks found that half did not know they were pregnant for at least two months, and a further quarter only discovered their pregnancy at three months or later. Two in five said that their periods had continued.

The findings, published today by the University of Southampton and the University of Kent, will reignite the row over whether the limit for abortions should be reduced from 24 weeks to 22 or even 20 weeks.

The campaign to reduce the limit is fuelled by medical advances, which mean that babies born at 22 weeks can survive.

About 11 per cent of the 191,000 abortions carried out each year come in the second trimester. Ministers say they are not convinced by arguments to reduce the time limit and believe that there is no consensus among doctors.

The research, involving 883 women, found that while ignorance of their pregnancy was the primary reason for women seeking late abortions, other factors often conspired to push back the termination further.

About a third admitted that they had delayed for more than two weeks before carrying out a pregnancy test.

Once they were certain, half of the women took more than a week to decide what to do — often because of concerns about the procedure and disagreements with partners.

Another delay came when the women saw their doctor. Almost two thirds said that a significant period elapsed between their requesting an abortion and having it, even though medical guidelines emphasise the need for urgency at this stage.

Two in five waited for two weeks, and nearly a quarter waited three weeks. The main reason for delay was confusion among GPs over where late abortions could be carried out.

Investment in services has been heavily concentrated in early “chemical” abortions for women up to nine weeks pregnant, which are available widely in hospitals and clinics.

Late abortion services have almost all been contracted out, partly because many hospital doctors are unwilling to carry out the procedure. There are only about a dozen clinics specialising in late abortions.

Anne Furedi, of the British Pregnancy Advisory Service, said: “We 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 and continue to use contraception. We know that many family doctors are not up to date with modern abortion techniques and give women unclear advice.”

The figures

191,000 British women had abortions in 2005

89 per cent of the total were carried out before 13 weeks gestation, and 67 per cent before 10 weeks

32 per cent had had at least one previous abortion

84 per cent of abortions are funded by the NHS

52 per cent are performed under NHS contract in independent clinics, such as those run by Marie Stopes and the British Pregnancy Advisory Service

Source: Department of Health

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