It is most often recommended in the 7th or 8th month of pregnancy. Since all of our bras are designed to grow with you through your pregnancy and nursing, you can choose to buy earlier in your pregnancy. All our bras have 5 sets of hooks and eyes. This allows you to adjust the band as your ribcage expands and contracts.
After your milk comes in and your breast have regulated (approximately 6 weeks post-partum), it is a good idea to re-measure yourself and update your nursing bra wardrobe, if needed.
A good rule of thumb is 3. You will wear one, have one in the wash and have one on hand if the one you are wearing gets a little too much of the let down.