Audio problem: While all four guys are on the bed watching the game, Richie's eating a sub so we hear Doug say, "Hey, Richie, come on," but Doug's lips are closed and not moving in this shot. (00:15:10)
Audio problem: When Doug goes to the door to call for Uncle Hank, the audio is over dubbed. (00:04:05)
Head First - S1-E6
Audio problem: Doug calls Arthur near the start and the audio is overdubbed. (00:03:15)
Road Rayge - S1-E9
Audio problem: Doug gets his license back and gets into his car. We hear the engine start, but his hands are seen on screen at the time. (00:20:00)
Answer: To answer the question directly. Carrie and Doug do not have a biological child during the show's main run. In season 3 or 4 finale (cant remember which) Carrie has a miscarriage. Later on in the show they find out Carrie has 1 working ovary. Thus reducing the chance of them conceiving. By the final season, they couple agree to adopt from China and go to collect their child in the show's final episode. While on the flight back from China it's revealed that Carrie is once again pregnant. However the last scene of the show is a flash forward showing that Carrie goes full term with that child and they are struggling with both kids.
Ssiscool ★
Adding on to this answer, I read somewhere that producers were afraid of the show having a "before baby" feeling and then an "after baby" feeling. Some fans feel that main characters, or the show in general, decline in quality after a major change. For example, Jesse and Rebecca on "Full House" after they had twin sons, and Daphne on "Frasier" after she married Niles.