Sex attacker Isaac Parker has been jailed for six years after being branded "a grave danger to the public".
Parker, 41, who has two previous convictions for attacking women, chose his 36-year-old victim as she walked home across Port Meadow, Oxford, in June.
Nicholas Syfret, prosecuting, said Parker knocked the woman to the floor and used his weight to hold her down. He then indecently assaulted her.
Mr Syfret said: "She recalled him saying, 'Shut up - if you say anything I'll kill you'.
"She, in a desperate attempt to stop a further assault, tried the strategy of saying she was pregnant and 'Don't hurt my baby'."
But Parker ignored her and demanded money and jewellery before fleeing. He pleaded guilty to robbery and indecent assault.
Mr Syfret told the court the victim suffered vivid flashbacks for a month. She now suffers from insomnia and anxiety attacks and has undergone counselling.
The court heard Parker was jailed for three-and-a-half years in 1989 after breaking into a woman's home at night and indecently assaulting her.
In 1994 he attacked a nurse at the Horton General Hospital in Banbury and was given 12 months' probation. Mr Syfret said Parker also had a string of previous convictions for violence.
David Bright, defending, said a psychiatric report on Parker, of no fixed address, was "not favourable".
Mr Bright said his client had given himself up at the police station shortly after the attack. Judge Anthony King told Parker his was a "very grave and serious offence" and added: "It must have been a terrifying ordeal for your victim.
"I'm satisfied the real reason behind this attack was sexual and only exacerbated by drinking. Further, I am satisfied that reports are essentially correct in the assessment of you as being a grave danger to the public."
The judge said he was imposing a longer sentence than normal to protect the public.
Parker was also ordered to serve 625 days left over from an earlier sentence because his latest crime was committed while on licence from jail.
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