Mammary Lane - S5-E5
Continuity mistake: At the beginning of the episode, the establishing shot shows a bowling alley with a sign that reads Mar Lin Do Lanes. A few minutes later, Doug welcomes Denise on behalf of the family at Bowl-O-Rama. (00:04:10)
Mammary Lane - S5-E5
Visible crew/equipment: When Denise is running away and Doug is on the floor, a camera shadow is very clear on Denise's car. (00:21:30)
Revealing mistake: In the final scene of the episode, Arthur is searching for used bottles of nasal spray. He empties the contents of the trash can on the kitchen floor and finds one after sifting through the trash. However, the bottle had already been positioned on the floor next to the trash can before he picked up the trash can so it would be easy to find when the trash was emptied on the floor.
Revealing mistake: When Carrie knocks the cards over at the start, some stay in the pyramid shape despite being knocked over, revealing the cards are glued together. (00:03:15)
Continuity mistake: When the IPS guys are discussing the Superbowl party, one of them takes a drink. He then lowers his can. When the angle changes, the can has changed hands. (00:04:20)
Continuity mistake: When discussing the Superbowl at IPS, Danny has a roll in his hands. Camera changes and he picks it up again. (00:04:30)
Visible crew/equipment: Just as Arthur walks in on a mobile phone, a boom mic is visible at the top of the screen. (00:04:00)
Continuity mistake: When Carrie takes the first lot of Hofferman's pictures off Doug, she's holding them portrait. Camera cuts and she's holding them landscape. (00:05:30)
Revealing mistake: When Deacon tries the doors on the van, the wall moves, revealing it's not a van but a set. (00:08:30)
Continuity mistake: The leaflet in Carrie's hand at the start vanishes instantly. (00:03:10)
Continuity mistake: When Carrie is on about having never spent that much on a vacation, she has a leaflet in her hand that is open. Camera cuts and now it is partially closed. (00:03:10)
Visible crew/equipment: When Carrie asks Doug to go and check out the smell in the basement, there are tape markers visible by the fridge. (00:05:20)
Continuity mistake: When Doug is being a polar bear, the bag Carrie was looking in disappears. (00:08:00)
Continuity mistake: When Doug asks Carrie to take a photo of him on the bed, there is nothing on the bed. After she says "stop", 2 items have appeared on the bed that were not in the previous shot. (00:08:45)
Continuity mistake: When Tim asks Doug if he wants to tee off back at the start of the hole rather than halfway down the fairway, he is pointing with his golf club. Camera cuts and he isn't. (00:15:35)
Continuity mistake: When the Marshall is chasing after Doug the distance between the two changes drastically. (00:16:00)
Continuity mistake: When Doug and Carrie are told that mold isn't covered by their insurance, Doug has his fist clenched by his mouth. Camera cuts and now it's open palmed and on the side of his head. (00:01:00)
Continuity mistake: When Arthur and Spence are talking in the kitchen, a fork keeps appearing and disappearing from Spencer's plate. (00:05:40)
Continuity mistake: When Doug and Carrie are talking about the fact the workmen have actually turned up, the cup on the table keeps rotating with no-one touching it. (00:12:15)
Visible crew/equipment: When Carrie hugs Doug and wishes him good luck, you can see the battery pack for her mic in her jeans. Clear as day. (00:17:35)
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.