NICOLE Richie will spend four days in jail after pleading guilty in a Los Angeles court yesterday to driving under the influence of drugs.

The reality TV star, who has not yet commented on reports she is pregnant, was also fined $2400 by a judge who said she was lucky no one was killed when she drove the wrong way on a freeway.

Richie, the adopted daughter of 1980s pop star Lionel Richie, was sombre as Superior Court Commissioner Steven Lubell lectured her in front of a court full of media.

"You are very lucky no one was hurt or killed as the result of this incident," he said.

"You need to understand that driving under the influence is extremely dangerous."

Richie, who wore a loose-fitting black dress, also agreed to serve a three-year probation term and enrol in an alcohol education program.

Richie, 25, was arrested on December 11 after drivers reported her car going the wrong way on a Los Angeles freeway.

She failed a field sobriety test and admitted to police she had smoked marijuana and taken the pain killer Vicodin earlier that evening.

Richie's plea deal came a month after her on-off friend and Simple Life co-star Paris Hilton was released from jail.

Hilton had violated the terms of her probation by driving after having her licence suspended for drink-driving.

Prosecutor Carolina Lugo said Richie's reported pregnancy had nothing to do with the plea deal.

"That was not discussed and we have no information about that," Ms Lugo said outside the courtroom.

Richie - who arrived at the hearing arm-in-arm with her Good Charlotte rocker boyfriend Joel Madden, also dressed all in black - was given the option of serving the 96-hour sentence in a paid city jail, which is seen as a more comfortable premises than a county jail.

She was given no surrender date, but she was told she must complete her jail time before September 28.

US authorities said if Richie was pregnant, they expected her to present herself at the jail soon rather than wait until the latest possible date to begin her sentence.

Richie, whose elegant appearance echoed screen star Audrey Hepburn in Hollywood classic Breakfast At Tiffany's, refused to comment outside jail about the sentence or rumoured pregnancy.

Last month, Richie told US chat show host David Letterman that she was "scared" to go to jail but ready to accept any punishment dealt by the court.

"I'm willing to face whatever consequences come my way," she said.

Richie has spent time in rehab for drug addiction and in 2003 was convicted of driving under the influence.

Commissioner Lubell told Richie she would spend a year behind bars if she violated her probation.