It is always advisable to purchase at least 2 nursing bras if you intend to breast feed, regardless of your breast size. The bra should have a zip or drop cup fastening, as this allows one cup to be undone at a time. Some women like to conceal breastfeeding, but it is important that your breast is totally free during a feeding session. With a normal bra, it means taking the entire garment off. It is recommended that a bra professional measures you for the bras, as a poorly fitted bra can cause problems such as mastitis (infection of the breast ducts). It is best to wait until about 36 weeks before choosing a bra, as your breasts will continue to grow. The average increase is about two cup sizes and 4 inches.
Ref: Ref; Expecting everything you need to know about pregnancy labour and birth 2004.
McGrail A, Metland D