To choose between breast shield sizes, a space of at least one millimetre between the nipple and the walls of the tunnel should be observed. Instead of opting for a correct size, it is essential to wear a larger breast shield. On the contrary, a breast shield that is too small causes cracks and pain. In addition to that, the amount of milk expressed is less. For this reason, the nipple should be measured after a feeding or pumping action. Usually, the diameter increases by 3 to 4 millimetres after each feeding or pulling.