Vaporeon should be the first one thats breast and belly expansion with breeding
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I didn't consider Vaporeon with belly expansion before! Could be a viable path for her.
There's a lot of hype for the first three Eevee evolutions triplets and I would want to design them better if possible. Perhaps I'll open a discussion for ideas about their growth paths and behaviors.
About the breeding part, I'm not sure if you're talking about pregnancy/egg-laid or just simply breast feed, as almost all the girls have breast feed. For the previous, I'm going to add them as additional story/scenes in the long future, but it's not my main focus for now.
For belly swelling, I have done slight cum inflation before on Nidorina, but I think it's not the same as impregnate.
I think the term is about being fertilized at start, seeing the belly swells larger and larger until eggs laids out right? Sorry that I'm not clear with the definition of the term. I tried searching the tags but it's a bit mixed with different ideas.