If you see a young pregnant woman who has made the slightest effort with her appearance, tell her she looks beautiful or wonderful or if you can't manage that, tell her she looks healthy. Having a baby is a big job and a great deal of responsibility that often comes with a high level of uncertainty. Make an effort and reach out to a stranger or an acquaintance. Pregnant women need to know that they are valued and beautiful.
You don't have to ask her when she is due unless she is known to you as this can seem like you are judging her as - too big, too small or too fat and certainly don't tell her any horror stories or tales of mishaps and failures. Just compliment her and wish her well. She will be glad you did.