Women who undergo a late abortion do not take the decision to do so lightly, a charity has said.
Women may not realise they are pregnant immediately
Marie Stopes International studied 100 women's reasons for having terminations between weeks 19 and 22 of pregnancy.
There have been calls to cut the legal limit from 24 weeks, but Marie Stopes said this would increase the hardship and distress for women.
In 2003, 1.6% (2,927) of abortions in England and Wales were carried out after 20 weeks.
Just under 90% were carried out in the first 12 weeks of pregnancy.
The British Medical Association voted recently not to back calls to reduce the current time limit for terminations.
The women in the study took part in face to face interviews or filled out questionnaires between January and April this year.
Many did not recognise the symptoms of pregnancy; either continuing to have periods or putting weight gain down to eating too much.
Others were taking contraception, such as the mini-pill, which meant their lack of periods was not unusual.
Some were aware of their pregnancy at an early stage, but were either in denial or subsequently experienced a change in circumstances, such as their partner becoming violent.
Others had difficulties getting appointments with healthcare professionals, while some encountered opposition to their wish to have a termination.
Liz Davies, Marie Stopes International's Director of UK Operations, said: "In recent months it seems that everyone - from politicians and medical professionals to journalists and anti abortion advocates - has been allowed to speak out on the issue of late abortion, except for those for whom it is most important; the women who have experienced it.
"We feel that this study goes some way to redressing that oversight.
"What emerges very clearly from this research is that no woman who opted for late abortion made the decision lightly.
"To suggest otherwise is to do these women a great disservice."
A spokeswoman for the British Pregnancy Advisory Service backed Marie Stopes.
She said: "Women aren't being irresponsible, and the vast majority are not in denial about their situation.
"Everyone's story is different, but they have a valid reason for their decision."
But Julia Millington, of the Pro Life Alliance, said: "Our opposition to abortion is absolute, a position sadly not yet shared by the majority of the public.
"When it comes to late abortion, however, we are convinced that the public is firmly on our side."
And a spokesman for anti-abortion charity Life said: "This report is a futile swim against the tide of opinion polls and research in favour of a lowering of the abortion limit."