Yes the nipples just pop up a bit bigger which is one indication there is a pregnancy going on. Usually its just enough to make you go wow the belly feels different :) when you rub it.
I'm thinking you have a dog but its true in cats too.
You might find this site interesting if its dog pregnancy you are following as it has a good article and great links on the bottom of the page
http://www.lowchensaustralia.com/breeding/pregnancy.htm
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