I'm 8 1/2 weeks and mine have not started getting any bigger (or if they have, it's only slightly bigger). They aren't really sore, either, unless I poke them. I'm just wondering when everybody else's started to grow. I'm only an A right now, so I'm pretty sure they're gonna have to get bigger at some point!
My breasts started getting larger pretty quickly into the pregnancy. I'm currently 21 weeks and I went from a 36D to a current size of 38DD. I can't fit into any of my old bras. It may be different for each woman. You may have a sudden growth spurt near the end. I'm actually bummed because I've gone up so quickly and I know that they'll get even bigger still once my milk comes in. I haven't gained that much weight either so that can't really account for my shift upwards.
With my first pg, I didn't get much bigger at all. This is pg #3 (#2 was a m/c)...I'm 6 weeks and they are filling out quite nicely I have to say...too bad they hurt so darn much though!!! I'm kind of sick of hearing it...but I guess it's true when they say that everyone is different.
Mine grew at the beginning. I started as a 34A and busted out of those by week 9 or 10. I've been wearing sports bras...which I highly recommend...especially the ones that stretch to accommodate multiple cup sizes. They offer great support and are fairly comfortable to wear to bed. Plus, they stretch with you, so you can wear them longer into your pregnancy before they don't fit anymore. I got several cheapies at Wal-Mart for about $4-$5 each, and they are holding up fine.
I ended up buying 1 real bra around week 15 or so because the sports bras have the tank top effect and didn't allow me to wear a lot in deeper v-necks. That real bra was size 36 nearly C (one of those half-sizes).
Now, at 23 weeks, I'm going to need another real bra...I'm sure I'm well into the C if not almost a D. Interestingly enough...I'm still wearing all the sports bras I bought initially. I save the "real" bra for certain outfits, and plan to wear the sports bras until they are worn out.
I think doing it this way has saved me plenty of $$ in bras in the long run.
I'm 22 weeks along and I went up a cup size almost immediately. Tomorrow I'm going to go get measured again for another new bra. Jenn's right though...it would be nice to keep them when it's all over with!