Responses
Some author-nim just want to make it a little different between pregnant female belly and pregnant male omega belly and maybe that's explain why pregnancy in omega male not noticeable during the first trimester.
[SPOILER]
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But don't worry. There will be scene where Gyuha's belly get noticeably big and his anxiety came again because he gain weight.
Also his first baby's kick scene. Gyuha's reaction is cute (≧◡≦)

why do they never show a big belly....