Visible crew/equipment: When Fisker comes down to the drivers area of IPS, there is a white T Mark on the floor. (00:14:20)
Cheap Saks - S6-E15
Visible crew/equipment: While Doug reaches for the phone when Trish is calling you can see where the kitchen wall is ending on the set. (00:11:53)
Visible crew/equipment: Jessica asks Carrie what number they live at - as she does so a tape marker is visible on the floor. (00:06:50)
Hungry Man - S1-E21
Visible crew/equipment: When Arthur accuses Spence of trying to push him out, there is a white taper marker visible on the rug where Arthur stands to accuse him. (00:17:50)
Hungry Man - S1-E21
Visible crew/equipment: When Spence is pacing there are tape markers visible on the floor where he turns. (00:17:20)
Hungry Man - S1-E21
Visible crew/equipment: When Doug explains he's working the double shift a white tape marker is visible on the floor by the front door. (00:03:20)
Visible crew/equipment: When Richie is explaining about getting rid of the car, tape markers are visible by Doug's feet. (00:14:40)
Visible crew/equipment: After the first drive in the Mini, a stage light is reflected clear as day in the bonnet. (00:13:20)
Road Rayge - S1-E9
Visible crew/equipment: As Doug takes Ray's keys, the film crew and equipment are reflected in the wing mirror. (00:09:50)
Visible crew/equipment: When speaking to Doug about the book, a crew member is reflected in the window. (00:19:35)
Visible crew/equipment: At the start when Carrie says says "it's already 20 of" look at the screen right to see the kitchen cabinets swing into place as the crew replace them. (00:00:35)
Visible crew/equipment: At the end in the theatre, a boom mic drops into shot. (00:19:00)
Visible crew/equipment: When Doug gets the beach ball, all the stage lights are reflected in it. (00:14:00)
Visible crew/equipment: When Carrie notices that Doug has walked out the room, there are some tape markers on the floor from where Doug was stood. (00:09:40)
Strike One - S3-E5
Visible crew/equipment: When Carrie is trying to return the truck, there are tape markers visible on the floor. They are little white dots. (00:11:35)
Visible crew/equipment: When the dummy is on the sofa with Arthur, the stage lights are reflected in its sunglasses. (00:16:00)
Visible crew/equipment: When Arthur is saying about how someone should be able to go out their house and turn left or right, he looks up and the top of the set and the set lights are visible in his glasses. (00:02:45)
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.