Continuity mistake: In front shots of the house you see that the garage is in the front and opens to the street, but the garage door is behind the house entirely. It seems to open in a back alley or private entrance way. The way it is shown in establishing shots, it can not be open to the front street at the front of the house.
Continuity mistake: In many episodes, scenes are shot on the front porch. It is often shown as a large porch that spans a great deal of the front of the house. But shots of the house exterior from a distance show a small set of stairs with very little space near the door. This isn't a porch on the back of the house as it has been shown to only be door to a small yard.
Continuity mistake: Outside shots of Coopers, upper half of each door has 9 windows. 3 sets of 10 windows on either side of doors. Inside Coopers, main doors are all glass and no windows on either side.
Continuity mistake: In episodes that show the outside of the IPS building, there are some shots that show a pay phone outside of the building, then other shots where the pay phone is missing.
Continuity mistake: The Heffernans' house is one of a row of identical brick houses. However from inside the Heffernans' living room, you can see out the side windows that the neighbour's house is beige with horizontal paneling.
Continuity mistake: From outside shots of Coopers, there are no garbage bins or newspaper boxes in front of the pub. From inside shots, several bins can be seen just outside the glass doors.
Continuity mistake: The garage door in the outside view has 4 windows. Switch to the inside scene and it has 3.
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.