Window Pain - S5-E2
Visible crew/equipment: When Doug gets into their Jeep after meeting the new neighbours, a man and the backstage can be seen in the passenger side mirror. When the shot cuts back to Doug again it's darker but when he backs up, the reverse lights of the Jeep reveal the man and stage again. (00:26:55)
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)
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)
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)
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)
Visible crew/equipment: During the drag race at the end, several times on the van you can see camera shadows. (00:19:10)
Visible crew/equipment: The second time Doug pulls up at the toll booth, stage lights and equipment are reflected in the car's paintwork. (00:07:20)
Dog Shelter - S5-E23
Visible crew/equipment: When Doug finds out that Rocky is in fact Rocky 4, a boom mic is reflected in the window. (00:10:20)
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.