Well, on one hand, it's a bit silly to expect public posts not to be read. On the other hand, she didn't explicitly say she was pregnant, and it is a bit weird to comment on what was (albeit easily) deduced, especially since you aren't that close. If she wanted you to comment on her pregnancy, she probably would have told you she was pregnant. (And FTR I do think it's a bit ridiculous to vaguebook on twitter about pregnancy.)