When you purchase a guinea pig from a pet store, they'll just replace it with another one from a breeder. If you get one through animal control or the SPCA, your guinea pig could be pregnant or might even have mites or a health condition, depending upon how they operate. They don't intentionally breed, though, so if you adopt from them, they won't breed more to replace them. If you get one from craigslist, you need to screen the person placing the ad carefully to make sure they're not a breeder. On the other hand, craigslist guinea pigs are often unwanted pets or accidental litters. Rescues take in unwanted guinea pigs. Reputable rescues will not adopt out sick or pregnant guinea pigs. If for some reason they miss a pregnancy, they'd take her back so she would have her pups in a safe environment with vet care, if needed. Rescues provide vet care for any sick guinea pigs and will take back the guinea pigs if you change your mind. They're your best bet.