When it comes to choosing a breast shield for a breast pump, one size does not fit all. A breast shield that is too small will squeeze your nipple, while a breast shield that is too large will prevent the shield from forming a good vacuum seal. Both cases will affect breast emptying and eventually lower your milk production. Because most breast pumps come with a standard 24 mm breast shield, you can use this size as a starting point.
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Moisten and then place the breast shield over your nipple, making sure your nipple is in the center of the shield.
Turn on the pump and let it run through a few cycles.
Look for signs the breast shield is too small. If the breast shield is too small, you will notice your nipple rubbing against the sides of the tunnel, you will feel the area around your nipple start to become sore, or you will see a ring of skin flecks inside the tunnel when you finish and remove the shield from your breast.
Look for signs the breast shield is too large. If the breast shield is too large, you will see the areola, or brown area around your nipple, move into the tunnel and you will observe little milk flowing into the pump.
Switch to the next size up or down and test again. When you have the correct fit, you will observe a pain-free, rhythmical motion in your breast with each pump as well as feel your breast begin to empty.