I knew (with 4) before I could even take a pee test. I remember with one, I was 99% sure the next day. It sounds like /that happened but when you pay attention to your body, you notice changes.  One weird thing is during my pregnancies, my sense of smell was intensely accute, & I enjoyed eating things I typically don't. I wonder how that affects a fat? It was weird, but I could smell when one of my employees had high blood sugar (type 1) & I made her go home.