ATTENTION EVERYONE QUESTIONING THE ANATOMY/BIOLOGY OF THIS,

There are multiple possible explanations for why she appears pregnant in her human tummy as opposed to her horsey bits:
1) Cerea may actually not know how it works/would appear herself, and this is her imagination.
2) It could be strictly for the reader's convenience; this visual instantly clicks as "pregnant" in our minds.
3) Quite simply, Crabman (Okayado) had limited space in the manga to fit the whole scene.

This fits in only half of a page (Ch16, p.22), and lotsa space was needed to accurately portray the backgrounds of the train and "off to work" scenes, as well as the lewd "welcome back". The anime recreates the original representation flawlessly, I must say.

A pregnant horse tummy can satisfy your needs…?