Pregnancy can go all kinds of ways. It can be thrilling or terrifying, peaceful or chaotic, joyful or fearful, or maybe all of those at different times. The point is, you can't know what a pregnant woman is experiencing. Assuming that someone is experiencing pregnancy a specific way and then forcing that assumption on them probably doesn't feel like a big deal when you're doing it, expect for the fact that you surely aren't the only one. When 10 people remark on the size of your belly, it's tiresome. There are subtle ways you didn't realize you're offending pregnant women, women who need bolstering and support, not critiques and opinions.
When I was pregnant with my last baby, I can't tell you how many times I heard how exhausted I looked (I was, and it was depressing to hear how much it showed) or how many times I had to politely deflect unwanted touches. On the one hand, it's beautiful that pregnancy brings out the caring, nurturing side of people. On the other hand, it's overwhelming that pregnancy brings out the critical, controlling side of people as well.
Using some consideration of this reality can make your next conversation with a pregnant woman go so much smoother. Just take a look at the ways you're offending pregnant women and try not to commit these blunders noted here.