I wonder if this is her first time in a dress? Especially a dress like that? She's not been a woman that long, so I can understand the uncertainty and the embarrassment.
The conflicted expression was intentional, yes. In addition to the long term worries you mentioned, she also has the immediate concern of being the center of attention while in such a state.
I dunno if the artist consciously and deliberately rendered her expression so that her smile looks equal parts natural and forced, but that's how it looks to me! She feels that maternal giddiness that her obviously and unconcealable state of pregnancy brings upon her -- an effervescent anticipation that's coupled with a nagging worry for all of the unknowns that come with being a first-time mother that she doesn't have any control over