I have always been a small c cup or full b cup and now I m busting out of my 34c bras. But attribute most of the growth to my weight gain. I was always skinny with no butt or boobs but I gained 13 lbs from week 8 to week 12. I have not gained any since then and I am nearly 17 weeks now. I don't want to buy new bras yet because I worry I will outgrow those also. So I m going to wait until I am further along and buy nursing bras whatever size I m at that time. Also having bigger boobs is a pain, they are starting to make my shirts fit funny and getting in my way. My husband is not complaining but I hope I go back down to my normal bra size after my baby is born.
They may just be fuller not necessarily growing in cup size.