Oh, you mean, when am I due with this food baby I am carrying? Right. The old adage, never ask a woman if she's pregnant unless you see the baby emerging from her nether-regions may be an overstatement, but it's a cautionary tale.
Commenting on other's weight-whether baby-related or not-is not appropriate in any setting unless you are actually said person's doctor/nutritionist/health specialist/etc. There is nothing like having a random stranger or mild acquaintance or family member even, come up to you and say, "Oh, TheDomina, have you lost/gained weight since I've seen you? With those judgmental eyes of shame upon me, I am forced to say, "oh, no, this outfit is just slimming/fattening/optical illusion" and run away from the conversation. For some reason when you are pregnant, people feel free to point out how large you have become, or if they believe you aren't gaining enough weight. I will tell you readers here and now, if someone is pregnant, it does not mean the baby growing inside them has sucked all of their personal insecurities and emotions out of them, they still do not welcome your comments on their girth.
Leaving the worst of all for last, asking the non-pregnant woman, when she is due. If you know someone intimately and they have told you they are pregnant, it is quite nice of you to inquire the due date and show a general interest in their well-being, however, do not ask the person you work with, haven't seen in a long time, or on the train/bus/etc if they are pregnant or when they are due. People come in all shapes and sizes and generally, we know what size we are, we do not need you to point that out for us.*
*Of course this rule does not apply if you fear a close friend or family member is ill with some form of eating disorder or other drastic illness that changes their weight. If you are concerned for someone's health, please tactfully approach them with your concerns.