"I owe the baby my life," she told Diane Sawyer on an interview aired on ABC's "Good Morning America" on Thursday. "I owe this baby everything and I have a responsibility now. I'm now responsible for someone else. I need to set the right example."
With her rocker boyfriend Joel Madden at her side, Richie explained what happened on Dec. 12, the night she was arrested for driving under the influence.
She said she was driving on the freeway near Glendale, Calif., a neighborhood unfamiliar to the 25-year-old reality star.
"When I was getting off the off ramp, instead of making a left to get on the street I went back on the off ramp," she said. "And I was in such shock that I just stopped the car. Then, within two seconds, police were there and we got off the freeway."
Richie admitted that she had been smoking marijuana and had taken the prescription painkiller Vicodin earlier in the day. She said she thought she was OK to drive.
"I never in my head thought that could definitely play a part in your driving," she said, as Madden shook his head and smiled. "Like, never did I think that … I'm so lucky that I didn't hurt anyone else."
Richie, who is four months pregnant with Madden's child, said that she doesn't do drugs anymore. She said, as she has said in the past, that she was taking the Vicodin for severe menstrual cramps.
"It's been a pattern in my life," she said. "When I get in trouble, I try to get out of it. That's what I've been doing, from the time I was little, and I really wasn't learning anything from that. I saw myself start to go down that road again with this particular situation."
It was when Richie and Madden were watching television and saw images of children who'd been killed by drunk drivers that she realized she had to make a change.
"At that moment I literally felt like the worst person in the world for even trying to get out of that situation," she said.
Richie pleaded guilty last Friday to the DUI charges and was sentenced to four days in jail.
She said that she's not afraid of serving her time, which must be completed before Sept. 28.
Richie's focus now on is her impending motherhood. She said that the couple's parents were "overjoyed" by the news about her pregnancy, although it was difficult for her to tell her father, Lionel Richie. She said that she had made many phone calls to him in the past when she was in trouble, so he was expecting the worst when she called to say she had something to tell him.
"I said, 'well, you're going to be a grandfather' and he was really quiet," she said. "Then he said, 'Now you're going to get to experience everything that I have and everything that you've done to me. It's definitely going to come back to you.'"
That may not happen, if Richie is able to meet her goals as a mom.
"I want my child to look up to me and be proud of me," she said. "I want to be the best parent I can be."