Nicole Ritchie pleaded guilty Friday to driving under the influence of drugs and was sentenced to about four days in jail and fined by a court commissioner who said she was lucky nobody was killed when she drove the wrong way on a freeway.

Richie, the 25-year-old daughter of pop singer Lionel Richie, was subdued and somber as she was lectured about her driving. She entered court on the arm of boyfriend Joel Madden.

Richie, who's rumored to be pregnant but has not confirmed it, wore a loose-fitting, knee-length black dress and spiked heels. She stood before the judge and softly answered "guilty" when asked how she pleaded to the misdemeanor charge. She initially pleaded not guilty in February.

She nodded when asked if she understood terms of the plea agreement (including 90 hours in jail and a fine of $2,048) worked out between her lawyers and prosecutors. Superior Court Commissioner Steven K. Lubell also gave Richie three years of probation and warned that she would go to county jail for a year if she violates it.

She must complete her sentence by Sept. 28. She can serve it in either a city or a county jail.

She was ordered to enroll in a drug and alcohol program and must prove that she has completed it when she returns to court on Feb. 20, 2008.

"You are not to drive with any measurable amount of alcohol or drugs in your system," Lubell told her. "You are not to drive without a driver's license."

He said the state Department of Motor Vehicles would decide whether to suspend her license.

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

Richie was arrested early on Dec. 11 after witnesses reported seeing her black Mercedes-Benz sport utility vehicle headed the wrong way on a freeway in Burbank.

The California Highway Patrol said they found her parked in the car pool lane.

Richie told authorities she had smoked marijuana and taken the prescription painkiller Vicodin, a CHP officer said at the time.

No drugs were found on her or in the car.

Lubell warned Richie of the dangers of driving under the influence and said: "The facts of this case, as the court knows it, is pretty scary. Someone could have gotten killed or injured. The court does not want anything to happen to you.

"If you continue to drive while under the influence of drugs or alcohol and someone gets killed, you can be charged with murder, not manslaughter.

"I hope that never, ever happens."

He said such a murder charge would come under a specific legal provision because she now has two convictions, the first one in June 2003 when she was convicted for driving under the influence of alcohol. The 2003 conviction made her eligible for a sentence of 90 days to a year in jail.

The reality TV star's lawyer, Shawn Chapman Holly, acknowledged to the court that the plea bargain reached with the prosecution was acceptable.

Richie made no comment to reporters as she left the courthouse, surrounded by private bodyguards.

Richie's jail sentence follows that of Paris Hilton, her co-star on the reality TV show "The Simple Life."

Hilton recently was released after spending about 23 days in custody for violating probation in an alcohol-related reckless driving case.