It is possible to breastfeed with most forms and sizes of nipples although in some cases latching on may be difficult and thus require some help during the first few days.
When nipples are flat but elastic like, the child will need to open his or her mouth wide and make sure most of the areola is inside the mouth. In this way, once suction begins a void is created and the nipple will protrude with suction. There are devices that can help to draw the nipple out.
In the case of “truly inverted” nipples; that is, when the areola is pressed down gently the nipple retracts or disappears, it is more difficult for the child to latch on but it is not impossible. It is important to receive help from and expert right from the beginning.