I don't believe people who say they had periods throughout.
I had bleeding every four weeks until month 6-7, it was as if my body forgot I was pregnant and decided to start up a period then remembered 'hang on, there's a baby' and stopped it again. If I didn't already know I was pregnant I would have presumed that was my period.
I know someone whose periods stopped so she did a pregnancy test which was negative. A month later and still no period so she did multiple tests, all negative. GP said these things sometimes happen, if you've had negative tests after this amount of time then you're certainly not pregnant, give it another month and then we'll look into it. The next month still no period, still negative tests. Back to the GP she went. GP took some bloods and then checked her over "oh dear, I can feel a mass in your lower abdomen" and referred her for an urgent scan the following day. She went to the scan expecting to be told awful news, instead she was told she was four months pregnant.
I have a friend who bled throughout pregnancy, did get a baby bump at all through a combination of a 5lb baby, strong abs, and the baby lying tucked right back against her spine. She had awful back pain but no bump at all. She also said that if she didn't know for a fact she was pregnant, she would have presumed she wasn't. When she was pushing the pram around with a newborn in it there were people who didn't realise the baby was hers because she hadn't looked pregnant.