I wondered

I think it as simple as that she has a serious mental illness. She laments that she has no children to her husband, so that can't be the explanation. Not that women usually go nuts when they are pregnant.

Having said that the movie is oddly inexplicable, and not in an interesting David Lynch kind of way either. What, for example, is the deal with her mother? Why is Marge the community's unpaid midwife? Why is Sonny pulling boards from the house? Why is the town 'importing' barmen in the Great Depression, and why is a new barman treated like royalty by the pub owner? How is it that there is a rooming house directly over the road from a hotel in a one horse town? What is the motivation for Sugar's betrayal of ALL of the 3 major characters?

Simply: a lot of this movie is incomprehensible.

While I'm here, the art direction, cinematography etc is wonderful. Neither Ward nor Vidler could act, at least at this stage of their careers. Brown is a subtle and powerful performer and is in a different league to everyone else in this production, including Neill. The soundtrack is godawful. Synths in a period piece set in the 30s? Even for a film made in the 80s that is a truly dumb decision. The script is stilted and uneducated in its language choices and cringeworthy at times.

I'm not fond of remakes, and generally think each generation should create their own stories, but this movie, I think, deserves another try.