It varies really. You might not even feel them at that stage, depends how in tune with your body you are, I think. Can be anything from feeling like all your tummy muscles have gone tight and being mildly uncomfortable for a moment, to full blown painful major tightenings that last a little while. They get worse the further along you usually, but you might be lucky and get non at all.
My first ones were like mini period type cramps that lasted a few seconds, but stopped as soon as I moved or stopped what I was doing!