"Yes I am. We are. I'm almost four months," she tells ABC News' Diane Sawyer in a new interview airing on Good Morning America Thursday and on 20/20 Friday.
Richie, 25, confirms that the baby's father is rocker boyfriend Joel Madden, 28, whom she's been dating since earlier this year.
In excerpts provided by ABC, Richie also opens up about her actions on the night of her Dec. 11 DUI arrest, and why members of young Hollywood are suddenly finding themselves in so much trouble with the law.
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Richie was sentenced to four days in jail. She also agreed to serve a three-year probation term, must enroll in an alcohol education program, and pay a fine of $2,048.
Reporting by SHARON COTLIAR
How did Nicole hide her growing tummy? See PEOPLE's photo gallery!