Is a DD equvilant to a E Cup?

I am trying to find a good nursing bra to order offline since I am on bedrest but everywhere I look online I cannot find anything decently priced (under $30) that is a 38DD which is what my size is now. Everything seems to be in the Single Letters. So I was wondering if I could just skip to a 38E instead and how much bigger would that be? Since I know I will be larger when I am nursing.