There is really no general answer to this that applies in all cases. It depends on a lot of factors, including what body shape and size you have normally, and what kind of clothes you’re willing to/normally wear, and who you’re hoping to hide it from, and how far you’re willing/able to go to try to hide it. There are always ways to hide it if you’re willing to go to extremes, in Victorian times people would just head off to some remote country estate and allow no visitors until the baby was born and they were ready to “return to society”. Of course that kind of pattern of unusual behavior can also reinforce any suspicions, so it’s always going to depend on who exactly you’re trying to hide it from and how much they already know about you or how much they might suspect that you’re pregnant.

A casual stranger on the street who has no context about who you are or what you looked like before your pregnancy and only encounters you a single time might have a much tougher time ever confidently identifying you as “pregnant” beyond a reasonable doubt unless it’s really obvious and it’s not always going to be obvious for everyone with every body type even in the late stages of pregnancy, it may not even occur to them. On the other extreme, if you’re living with and trying to hide it from your mom, or your partner, or even a close friend, and they’re vigilant for the possibility that you might be pregnant and are paying careful attention? Good luck, they’re going to figure it out sooner or later and probably a lot sooner than anyone else because they have so much familiarity with you as a person, and even your habits and behaviors, and they might even start noticing some changes intuitively even if they aren’t visibly obvious, including changes that you might not even have noticed yourself. And that just snowballs into more careful attention and analysis until they figure out what is probably going on with you. Sometimes they might even know or suspect it before you do. It happens.