Basically leave it as late as you possibly can, 38+ weeks or so. You may not feel like it, or have the time once LO is here and once your milk comes in a couple of days after birth you will want them straight away. I think I got mine about 38 weeks last time. Yes, you will go up a bit when your milk comes in and your boobs swell like mad, but that will subside again once you settle into feeding so your size at 39-40 weeks won't change too much. By their nature, maternity and nursing bras are flexible and stretchy anyway so you should be fine.