Window Pain - S5-E2
Continuity mistake: When Doug and Carrie are arguing outside the neighbour's house, Doug takes a big swig from the Scotch bottle and then puts it on the ground. After the police pull up, the shot cuts back to Doug and Carrie. Doug picks up the bottle from the neighbour's porch railing. (00:42:45)
Continuity mistake: In the middle of the night when Carrie finds Doug rolling the ham-n-swiss doobie in the kitchen, he dips it in the mayo, and when the shot changes, there's no mayo on it and he dips it again, then takes a bite. (01:25:00)
Continuity mistake: When Doug is talking to Carrie in the kitchen about his ideas for the sandwich shop, lights reflecting in the living room windows behind Doug turn off and then back on when the shot changes between Doug and Carrie. (00:11:00 - 00:11:35)
Continuity mistake: When Doug jumps out of the porta-potty before the rival ice-cream truck smashes through it, the door is open and Doug immediately lands on the ground; but when the shot changes, the porta-potty door is closed and we can't see Doug rolling onto the ground. Also, when the truck smashes into the porta-potty, liquid splashes everywhere; but when we see Doug in the next shot he is dry and no liquid on the ground around him. (00:12:55)
Party Favor - S2-E13
Continuity mistake: The colour of the card on which Doug the first draft of his speech changes from blue to white and back to blue. (00:09:00)
Continuity mistake: When Doug and Carrie are in Deacon's kids' room, Carrie is changing the baby and between shots the baby changes from lying down to sitting up. (00:15:00)
Inner Tube - S3-E17
Continuity mistake: At the start at IPS, Doug's tie is looped over itself twice. Camera cuts and now it's not looped at all. (00:02:30)
Continuity mistake: When Doug and Deacon are talking about Doug wearing a disguise to the concession stand, There are two people on the loading bay holding a parcel. Camera cuts and this parcel is no longer in their hands. (00:13:00)
Continuity mistake: Deacon says "How do we know cutie-pie isn't cutie-guy?" then crosses his arms. In the next shot his arms are immediately uncrossed and on his hips when Doug replies. (00:06:00)
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.