I had a couple of weird experiences........with my second baby, I had three normal periods (go figure) and then the rest of the (surprise) pregnancy went on as usual! The first two boys were 14 months apart!
With my fourth pregnancy, we had been VERY careful (40 years ago--no pill) and after four months with no period, I went to the Doc to see what was wrong.....and discovered another baby was on the way. Turned out to be our first girl! (Now Elderette!)
The last one (no periods 4 months) turned out to be a grapefruit-sized tumor which I left AT the hospital, LOL!
So mine were strange experiences, but I'm sure that if they'd been full-term. I'd have realized it was a baby long before the nine months was over! BTW, I weighed under 100 pounds soaking wet before every one of my pregnancies, actually the ONLY time I weighed-in @ over 100 lbs WAS when I was pregnant!
Those days are long gone! <grin>
Annie