If you need an x-ray, MRI, CAT Scan, Mammogram or other medical procedure, most folks can use monofilament. This is essentially a plastic piece the same thickness as your jewelry which is inserted in the piercing to keep the hole open. Many women leave their body jewelry in unless there is discomfort due to the changing shape of the body. At that time monofilament can be inserted which will be more flexible and comfortable. If there is no discomfort, you can leave rings as they are.
Milk ducts are a multiplicity of pore-like openings.
The feedback I've gotten, is either that it doesn't change anything at all, or that milk comes from the piercing sites, as well as the milk ducts. Most women do remove their jewelry to breast feed, though others replace their usual jewelry with something very small. This is the type of material often used in hip and joint replacements, and surgical screws. It is silver in color, but is NOT silver. Sterling silver tends to tarnish, and that is essentially a poison to your body.