Nursing bra size?

CD

I'm currently 36 weeks and my 36DD bra hasn't fit me comfortably for a couple weeks now. I am shopping for nursing bras and curious to know if I should buy 36 or 38? Will a 36 fit me after I give birth? I would hate to buy a 38 and it be too big once my pregnant tummy goes down some. What have been you all's experience?