nursing bras

Emily • I'm 28 my husband is 31. I am a FTM after trying for 15 months! Due the beginning of April with a little girl!
How do you know what size nursing bra to get? Before I got pregnant I was a 34A. I have since moved to a 36B. I'm 30 weeks pregnant and this  size is starting to become too small. I was thinking about just buying a nursing bra instead of a C and then a nursing bra. How do I know what size though since I don't know how big I will be when my milk comes in?