The concept art does show that in the early production there were ideas that Katara would be pregnant during the movie. I saw several Kataang fans go "WOE IS US WE WERE DENIED CUTE PREGNANT KATAANG!!!"
And like, are you brain dead.
What you want is: Katara, obviously and visibly pregnant, forcibly side lined for the entirety of an action movie, incapable of fighting or participating in the plot in anyway besides providing emotional support to Aang? All so that way you could maybe get a cute cuddly pregnancy picture? All the cool fight scenes we got in the movie with Katara? Her active involvement? Would be completely nixed if she was pregnant. And we're not talking first trimester either, she is obviously late term in the concept art. Furthermore, Aang's actions in the movie would become incomprehensible, stupid, and selfish, if he was actively endangering himself, his wife, and his unborn child with how he was acting. He is already acting that way in the movie but at least it doesn't borderline on full on dead beat dad before the child is even born!
(And I also just want to say: Katara's main function is not to be an incubator for the next generation of Air Nomads. Having her being visibly pregnant with what may or may not be an Air Bending child would have 100% changed the vibes here. Like, if the plot is about the next 'air nation' and what that will look like, Aang's potential heir being in the womb incubator of his wife would have to be apart of the conversation. I do not want a plot like that and you should not want a plot like that).
Furthermore, also consider this. At no point in the production did anyone ever apparently consider that Toph could be pregnant too.
What, you ask? Why would Toph be pregnant?
Well, Toph had two daughters who are around the same age as Katara and Aang's children. I'm sure you could parse the timeline but if it's about the time that Katara is having children... It is also about the time that Toph is having children. So, why was it never considered that Toph might be pregnant during the movie?
Because what? It's Toph? Gravid with babies is not how we visualize her? Toph gets to be an action hero and have cool fight scenes. Katara emotionally supports her husband and is considered for pregnancy. Why is that? What does it say about our (and the perception of the writer's) view of women, that we can imagine Katara pregnant but not Toph? Even though we know both of them, canonically, were pregnant and had children? Why is Katara considered for depictions as eight months pregnant but not Toph?
If I have to watch Katara subordinate her own feelings and wants to provide emotional support to Aang, the least Bryke could do is not also make her pregnant while doing it. Jesus.