"The routine use of restraints on pregnant women... [is] a cruel and unusual practice that can rarely be justified," the group said in a report.

Only one state currently bans leg irons on female inmates while they are being taken to hospital during labour.

US government policy urges measures to ensure foetuses are not harmed if a pregnant prisoner is restrained.

The New York Times described the case of one pregnant inmate whose legs were allegedly shackled together during 12 hours of labour, despite requests by a doctor and two nurses that she not be restrained.

"The doctor who was delivering the baby made them remove the shackles for the actual delivery at the very end," lawyer Cathleen Compton told the newspaper.

Shawanna Nelson had been jailed in Arkansas for identity fraud and writing bad cheques. She gave birth in 2003 at age 30, the newspaper said.

She is suing the prison and Correctional Medical Services, claiming she suffers ongoing back pain and damage to her sciatic nerve because she was largely unable to move during her labour.

The defendants deny having harmed Ms Nelson, the New York Times said, citing court papers.

Arkansas defends its policy.

"Though these are pregnant women, they are still convicted felons, and sometimes violent in nature," Dina Tyler, a spokeswoman for the Arkansas Department of Corrections, told the newspaper.

"There have been instances when we've had a female inmate try to hurt hospital staff during delivery."

Prosecution for rape

Amnesty also called for new laws that would cut down on the sexual abuse of women in prison.

"Statutes should bar sexual contact between staff and inmates and leave no room for exceptions," Amnesty said in Abuse of Women in Custody: Sexual Misconduct and Shackling of Pregnant Women.

The human rights group carried out a survey of law and practice in all 50 states, the District of Columbia and the US Bureau of Prisons.

Each state makes its own regulations on how prisoners are treated in its correctional facilities.

Amnesty found that not a single state had laws covering all six areas it considered essential for protecting prisoners from sexual misconduct.

It recommendations include:

Forbidding sexual relations between inmates and prison staff. Six states do not have laws barring them

Banning all forms of sexual abuse, including threats

Ensuring that laws designed to prevent abuse apply to all staff and contractors working at all correctional facilities and locations

Making it impossible to hold an inmate criminally liable for engaging in sexual conduct. At least one state has laws under which a prisoner can be charged for being raped.

It also proposes that female prisoners be guarded only by female officers and that pat-down searches of women be carried out only by women.