Okay, it's an issue when one is not pregnant, and an even bigger issue when one is.
I professed in my former vanities blog that I was like every other regular girl on the planet who'd always assumed my bra size would stop changing upon achieving full body development/maturity.
I used a semi-traditional method of measuring myself when I first bought my pregnancy bras. By "semi-traditional" I mean I measured my rib cage, measured my bustline, calculated the difference, and started fitting bras. This same method is the one described here on this site.
I know some women do the whole plus-minus thing. I don't because I really couldn't understand why. Anyone who knows the reason why is free to leave a comment to explain this to me. Please.
The method worked quite well for me, to be honest, and pre-pregnancy, I enjoyed the utmost comfort in my undergarments. And then the early pregnancy body changes.
Here's a new method I found via BabyCenter.com. "New" in that it adds a third measurement: the chest. Their instant Bra Size Calculator will do the job for you. I found a similar one here. It came up with a very different bra size for me compared to the bra size I would normally get from the method I mentioned using before though the HerRoom.com site does say that their method becomes unreliable with bigger sizes and while I don't fall under the bigger sizes they mentioned, my sizing has become...well, strange. In my opinion anyway since my rib cage didn't grow an inch bigger but everything else did. Again, if you're reading this and had the same experience, please let me know I'm still normal!
I need to go in for another round of shopping soon and I think I will give both measurements a try just to see which one fits me better. To be honest, I like the Baby Center measurement a great deal better because the size that came up, while still larger than the average, is a more common size and therefore easier to find.
And just for a little extra knowledge, here are Baby Center's Six Signs of a Good Fit and Five Signs of a Bad Fit. Also, if you're at that point I was at before--whining about having to spend hours finding a good fit again after already finding my perfect match--here are some basics on Maternity and Nursing Bras.