A woman who was kidnapped, raped and held captive for 18 years was so afraid of her abductors that she never tried to escape, according to court documents.

A convicted sex offender and his wife have pleaded guilty to kidnapping and raping a California girl in a surprise plea deal that will spare their victim and the two daughters she had during her 18 years of captivity from taking the stand at a trial.

A couple charged with kidnapping an 11-year-old girl and holding her captive for 18 years have made full confessions, a defence lawyer said.