Tempted to borrow your best friend's breast pump, or pick up a preowned one online or at a yard sale? Think again. Buying a used electric personal-use pump or borrowing a friend's can put your baby's health at risk because breast milk can carry bacteria and viruses. These potential contaminants can travel through the tubing and lodge in the pump's internal mechanism--the part that connects to the tubing--which can't be removed, replaced, or fully sterilized. With each suction and release, these contaminants can be microscopically blown into the milk you're expressing and possibly infect your baby, says Nancy S. Mohrbacher, a lactation consultant with Hollister Incorporated in Arlington Heights, Ill., the maker of Hollister and Ameda breast pumps. That's why manufacturers encourage nursing moms to think of a breast pump as a toothbrush or lipstick--like any personal use item you wouldn't share with a friend.

However, hospital-grade rental pumps, such as Hollister's Elite, SMB, and Lact-e Electric Breast Pumps and Medela's Symphony, Classic, and Lactina, are designed for many users and are built to last for years. They may prevent cross- and self-contamination with a special collection filter that prohibits milk from entering the internal diaphragm. Or they're designed so that the milk comes in contact only with the bottles and tubing that attach to the pump, so there's no cross-contamination.

The Purely Yours Breast Pump by Ameda is the only personal-use pump on the market to date that features a patented silicone diaphragm that provides a barrier so there's no air exchange between the pump tubing and the breast flange--so you never have to clean the tubing. The diaphragm protects expressed milk from contamination that may exist in the pump and protects the pump from any contamination in the milk. It also prevents self-contamination, which can occur when moisture and/or milk particles enter the pump tubing and organisms, such as bacteria and mold, grow there and get blown back into the milk. (The risk of self-contamination is why most other brands of pumps will instruct you to examine the tubing after every pumping and to clean or replace tubing if you see milk or moisture there.) Still, even with its unique milk collection system, the Ameda Purely Yours Pump isn't marketed as a multi-user pump because it's much lighter and more portable than Ameda's rental pumps. These have the same internal diaphragm as its personal-use pumps, but with a heavier-duty, "industrial-strength" motor, says Mohrbacher.