While I agree in principle that unhappy feelings about pregnancy are as "valid" as "Gee, this is terrific!", I also agree with the principle that casual conversations should not delve deeply into your personal emotional distress. Casual conversations are why weather was invented.
If the woman is truly unhappy about the pregnancy, or even just ambivalent, one hopes she has friends who she can discuss her feelings in a good long heart-to-heart without having them dismiss them as "Oh, after the baby arrives, things will be great." However, when you're just chatting it's rather unfair for them to dump heaping portions of angst on your plate, when you just know that if you repeated it to someone else, she'd be all over you for "gossiping about things that don't concern you in the least!"