A woman's body undergoes a lot of changes during pregnancy and even after it. You should be prepared for some drastic changes in your body when you are pregnant. It is during the 2nd and 3rd trimesters that you discover that your breasts don't fit into your bras. Obviously, you have to go for a change in bra-size. A good nursing-bra is one that is flexible enough to accommodate all the changes your body undergoes during pregnancy and even after delivery.
The ideal nursing-bra should have ample space for adjusting the change in shape of your breasts during your pregnancy. You might need a different cup size while you are pregnant, and another while you are feeding your baby. Your nursing-bra should be such that it can adjust to the changes.
You are going to need to feed your baby after delivery. So, your nursing-bra should have its cups adjustable to suit the purpose of breast-feeding. They should be aligned in a such a position that they can be opened and closed easily. The flap should be such that you can manipulate it using only one of your hands. The opening should be big enough to provide room enough for the baby to feed. The nursing-bra should never squeeze the breasts in any manner.
Your breasts are bound to grow in size till the time you deliver your baby, and it is not going to pause till you begin to lactate. When your baby has comfortably adjusted to your milk, the breast size is going to stop changing. So, you are going to need a nursing-bra as soon as you conceive.