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.
As for her guilty plea in the DUI case last Friday, Richie tells Sawyer: "I have a responsibility and it's something that I did wrong, and if I could personally apologize to every single person that has lost a loved one from drunk driving I would. And unfortunately I can't, but this is my way of paying my dues and taking responsibility and being an adult." 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.