Lol well I was a bit underweight when I fell pg with DD1 so I started showing pretty early on. I didn't lose all the baby weight so I was nearly ready to pop with DD2 before people started with the, 'Oh heeeyyy, you're pregnant!!'... I guess everybody thought I was just stacking it on and didn't want to say anything for fear of being rude

So yeah, I think it depends on the baby, the gap between kids, how much weight you put on/lose with/after pregnancy... but generally yeah I've found people tend to show a bit earlier with subsequent pregnancies if they've lost the baby weight before falling pregnant again