I think it really depends on the woman. Lots of fanfic has the woman suddenly throwing up and thinking she has the flu. But when I was pregnant, I didn't throw up even once the during the whole time. The woman's breasts could start to become tender - maybe someone (watching very closely) might notice her trying to shift in her bra or rub quickly at some of the soreness. With her second child, my friend's waistband started getting tight within a month, but I was still wearing belts while carrying my first well into my fifth month. So honestly, some women could hide their pregnancies fairly easily until they start showing in the second trimester.
