Continuity mistake: Right at the start, the salad cutlery keeps crossing and uncrossing itself. (00:01:05)
Continuity mistake: When Doug and Richie are waiting for Spence at the pub, there is a pepper shaker in front of Doug's beer that disappears and reappears. This happens again again after Spence arrives and Rickie is making fun of him. (00:04:15)
Continuity mistake: When Doug and Ritchie are waiting for Spence, the amount of beer in Doug's glass keeps changing. (00:04:30)
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.