Nicole Richie & her baby daddy Joel Madden, sat down with ABC News' Diane Sawyer ... they talked frankly about the rumor that has been buzzing for some time now.
Nicole finally addressed the nagging question: Is she really pregnant?
She says: "Yes I am. We are. I'm almost four months."
Nicole & Joel are expecting their baby by January 2008 ... they've been dating since the beginning of this year.
Nicole also talks about her DUI arrest & her 4-day sentence.

It seems Nicole really wants to grow up & start acting like an adult ... she tells Diane 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."

The interview will be airing on Good Morning America today and on 20/20 Friday.