I came up with this rule a long time ago. I have stuck to it. Not everyone follows it. I don't normally like to suggest what people do or don't say but I believe this is very important.
Women can have medical conditions that result in them appearing to be pregnant. There may be several conditions - I know of a few. One is fibroid tumours. I know about them from personal experience. I also know the pain of being asked if I'm pregnant. And of people looking at my stomach without asking. No, I'm not pregnant. If you only knew how loaded an issue that is to me and how much that question hurts, you would never, ever ask it. Or look at my stomach for too long. That hurts too.
Sometimes, a person is pregnant but it's not going well. It's quite possible she doesn't want to talk about it. Too many pregnancies don't end with the arrival of a healthy baby. Too many of us know about that.
Please, be considerate. I know we all get excited about babies and pregnancies but a bulge in a woman's belly, big or small, isn't always something about which they are happy and to which they want attention drawn. Sometimes things aren't how they look.