Foetuses cannot feel pain because it requires mental development that only occurs outside the womb, says a report in the British Medical Journal.
The debate about whether foetuses feel pain is long-running
Dr Stuart Derbyshire, of the University of Birmingham, said a baby's actions and relationships with carers enabled it to process the subjectivity of pain.
Pro-life groups say foetuses respond to stimuli from 20 weeks.
The US is considering legislation to make doctors tell women seeking an abortion it will cause the foetus pain.
It is also being suggested that, if the pregnancy is over 22 weeks, foetuses should be given pain-relieving drugs.
Dr Derbyshire, who is linked to pro-choice groups, said there were various stages of a foetus' gestation at which certain parts of the body's pain "alarm system" developed.
He concludes that pathways in the brain needed to process pain responses and hormonal stress responses are in place by 26 weeks.
But he says the crucial factor is the environmental difference between the womb - where the placenta provides a chemical environment to encourage the foetus to sleep - and that of a newborn baby, who is exposed to a wide range of stimuli and environments.
"Pain is something that comes from our experiences and develops due to stimulation and human interaction.
"It involves concepts such as location, feelings of unpleasantness and having the sensation of pain.
"Pain becomes possible because of a psychological development that begins at birth when the baby is separated from the protected atmosphere of the womb and is stimulated into wakeful activity."
Dr Derbyshire said whether or not foetuses felt pain did not affect the abortion debate because it did not change the moral viewpoints of the pro-choice and pro-life lobby, or the legality of terminations.
But he said: "Avoiding a discussion of foetal pain with women requesting abortions is not misguided paternalism but a sound policy based on good evidence that foetuses cannot experience pain."
And he said giving foetuses painkilling drugs - which would have to be given through the placenta - involved procedures which may expose the woman to unnecessary risks and distress.
But Anna Pringle, a spokeswoman for the pro-life charity Life, said: "This complex debate has been ongoing for a long time.
"Of course, if the unborn child can feel pain - as has been suggested by other research - then it makes abortion all the more horrifying.
"What we know for certain is that by the age of 20 weeks unborn children can respond to external aural stimuli such as music and conversation, and the 4-D ultrasound techniques used by Professor Stuart Campbell have shown that a child also responds to physical stimuli."
But she agreed that the issue of whether foetuses felt pain was irrelevant to the abortion debate.
"A human person is a human person, and as such has the right to life, which is quite literally fundamental to all other rights."