I've talked about this before, but didn't have any luck finding one of the threads, so I'll just go ahead and do a little repeat performance.
That is exceptionally rare. Not only it it rare, it really only tends to happen in a couple limited and very specific circumstances, like, when someone is:
• so traumatized -- like from ongoing abuse or a violent assault -- that they simply are no longer paying attention to their body at all.
• very out of touch with reality, like because of serious and ongoing substance abuse/addiction, or severe and untreated mental illness.
• deeply uneducated about their body and pregnancy, to the point they do not know things like that no longer menstruating (when it's not about menopause), having had intercourse, being very tired, and showing are about a pregnancy.
• simply being dishonest, something that happens more often than any of these other scenarios, and is particularly common in situations where, for the pregnant person, it doesn't feel safe in some way to be honest about having had sex (or with whom) or having been assaulted, going without contraception, or something else. Sometimes that isn't about safety so much as someone just not wanting to take responsibility.
And that's about it.