How can it tell you anything? The midwife doesn't know what you looked like pre-pregnancy!
I've got a very rounded stomach that you could measure as a bump if you were so inclined, but at 19 weeks I know that it's not that much bigger than normal - how would the midwife be expected to know that though? It's just where I've always carried my weight... (I'm 5'10 and size 14 with slim arms and legs).