Everyones body does different things, and from personal experience I can say being fitted in early pregnancy properly by someone experienced doesnt necessarily mean the bras will fit later on and after your milk comes in. I went up 5 cup sizes and my sis only went up 1.

I started out in a 12C before pg, a 14E eventually during (or 16DD), and ended up being a 12G after my milk came in. Its not only your bust that increases during pg its around your chest are you belly comes out.

I only bought a couple of cheap bras on sale while I was pg, sometimes not even maternity if I found wireless cheap enough, and even a sports bra that fitted well. I found the cotton bras like the Bonds bumps fitted better for multiple sizes because of the stretch and shape of them. After my milk came in I did have to resort to getting bras fitted from a local maternity shop.