@stokesm21 I'm a ftm, so take this fwiw, but I have been told by two independent but reliable sources (nicer bra shop and a lactation specialist), that the average woman goes up one band size and one cup size from where they were pre-pregnancy thru the end of pregnancy, and will maintain this new size while breastfeeding (so 34B - > 36C). They will be a little bigger the first few weeks as milk comes in, but settle to this size for a bit, then usually shrink back down to pre-pregnancy size when they ween. Again, this is the *average* woman. Mine haven't grown quite the full cup, but I did go up a band size. So I plan to get nursing bras that are about the size I am now in 3rd trimester.
I have no idea what my current bra size is! I need to get measured asap and get these nursing bras.
Like many of the PPs had said, I also used nursing tanks or camis under regular clothes and pulled my shirt up and the cami down. I like the nursing tanks from H&M, Old Navy & Nordstrom. For nursing bras I liked Jessica Simpson, Cake, Cosabella and Hot Milk. I honestly had a really hard time finding nursing tops that I liked, so I used the pull up/pull down method.