^(^) idk if Hachi's even pregnant in this picture (btw is that a spoiler?) but:
In physiological terms: pregnancy corsets were not just a thing, but persist into today in the form of maternity stays. Whilst I wear corsetry I speak with exactly 0 (zero) pregnancy experience, but women using them generally report them as comfortable and helpful (Proxy), especially when bustier. Rule of thumb: the mother's pain becomes unbearable before it can harm the child.
In terms of fashion: I do agree that this "shoulder-free bodice"-look with detached sleeves is ... unique, and this applies to both Nanas. But then the series features way worse.