It is unlikely foetuses feel pain until the last few weeks of a pregnancy, a review of medical evidence concludes.
The study looked at what stage of pregnancy a foetus could feel pain
The paper, in the Journal of the American Medical Association, was prompted by proposed federal legislation in the US.
It would require doctors to tell women having an abortion from the 20th week of pregnancy that the foetus felt pain.
But the University of California team said foetuses probably only have that capacity at 29 to 30 weeks gestation.
A full-term pregnancy lasts around 40 weeks.
The researchers say there is only limited data available on this issue.
But, writing in JAMA, they say pain requires the conscious recognition of an unpleasant stimulus.
This cannot happen until certain brain structures connecting the thalamus and the cerebral cortex develop during the third trimester of pregnancy.
These connections are not usually apparent until the 23rd week of pregnancy and may not begin to be made until the 30th week.
'Risks for women'
The team, led by Dr Mark Rosen, said: "Withdrawal reflexes and hormonal stress responses present earlier in development are not explicit or sufficient evidence of pain perception."
They add: "Because pain perception does not function before the third trimester, discussions of foetal pain for abortions performed before the end of the second trimester should be non-compulsory.
"Foetal anaesthesia or analgesia [pain relief] should not be recommended or routinely offered for abortion because current experimental techniques provide unknown foetal benefit and may increase risks for the woman."
But the team called for further research into the issue.
A UK expert said the JAMA conclusions were in line with what was already known about foetal pain.
Charles Rodeck, professor of foetal medicine at University College London Hospital and spokesman for the Royal College of Obstetricians and Gynaecologists, said that in all abortions carried out in the UK beyond 22 weeks, the foetus is either injected with something which stops its heart, or the mother - and therefore the foetus - is under a general anaesthetic.
He added: "The basic neuroanatomy must be there, and has to function, for pain to be experienced."
But Julia Millington of the UK's Pro Life Alliance said: "It is not the ability of the victim to feel pain that makes killing objectionable but rather the violation of that individual's most basic human right, the right to life."
Among those who contributed to the research were a medical student who has worked for an abortion rights group and the director of a clinic that provides abortions.