I'm a little behind you (23 weeks) but struggling. I had an online fitting the other day and have ordered a bra online, crossing my fingers it fits.
I've got a couple of tips that I've gathered.
If you look at the bras online, good ones will have 6 rows of hooks, so there's lots of room to grow. Everyone grows differently in pregnancy and once your milk comes in, so there's no guarantees how long a bra will fit you. But if you have 6 hooks then you have twice as much growing room as a regular bra! And, even if you grow out of this bra, you should be able to fit back into it again in the future after the baby is born.
It's recommended to have wire-free bras for the beginning of nursing. This is not a problem for me as I'm already used to the comfort of wire-free. But if you're hoping to wear this bra AND breastfeed, then it may be worth going for wire free.
Now the most crucial bit of advice! I was told that nursing bras, they fit you to the tightest hook (to ensure you have room to grow). Whereas normal bras, they fit you to the loosest hook so that you can tighten the bra as it stretches over time. This turns out to be wrong based on my first online bra purchase!
I was a 34D before pregnancy, and had to stop wearing my now-far-too-tight bras thanks to rib pain (thank goodness for stretchy crop tops!). I knew my band size had increased, and my cup size seemed to have increased as well (but not problematically). I measured myself three times, once with the help of this online consultation, and I was coming out as a 36D. Apparently this means I'd increased by one size in both the band and the cup. So I ordered a bra in this size. This ONLY fitted me on the loosest hook! So definitely not fitted to the tightest hook, and no room to expand further! So what I've done is returned it and asked for a 38D as an exchange. I measured the size difference between the tightest and loosest hooks, and it was about 2 inches. So I figured that if I was fitted to the tightest hook, it would be about 36D as I'm currently measuring, and leave me room to grow. I appreciate the cup might be a bit big, but I was filling the 36D cup so any further expansion would send me up a cup size too! I'm hoping this single bra will get me through the rest of my pregnancy (it has drop down cups so will be ok for nursing if I can still fit into it) and if it doesn't, well at least I only bought one. Just waiting on that exchange to see if it fits....fingers crossed!