Here is something that I posted on another thread some time ago.

Not spaying a dog before her first heat increases the likelihood of cancer, and letting your dog breed GREATLY increases her risk of cancer. As for weight, it will change absolutely nothing except the fact that your dog will be alot fatter for the months that it's pregnant, and about a month or two afterwards.

~An estimated eight to ten million dogs are euthanized in shelters each year. Millions are abandoned, only to suffer from illness or injury before dying. That's about one every 4 seconds.
~Most of the dogs in shelters are from backyard breeders.
~Over 56% of dogs and puppies entering shelters are killed. (Most of these dogs and puppies go to "good" homes).
~It costs U.S. taxpayers an estimated $2 billion each year to round up, house, kill, and dispose of homeless animals (So you may be adding to the taxes by breeding).
~For every human, there are about ten dogs. There are about 6 billion people in this world. Should every person own ten dogs?
~As many as 25% of dogs entering shelters each year are purebreds.
~Every unspayed female pet potentially contributes to the problem.
~A female spayed AFTER sexual maturity has a 70% chance of developing mammary cancer. By having a litter of puppies, her chaces of cancer are increased greatly by even more.
~Contrary to popular myths, there are absolutely NO benefits, medical or otherwise, in letting your female dog have even one heat cycle or litter before she is spayed.
~Your spayed female will be healthier. She will avoid the risks of pregnancy and uterine infections, and she will be far less likely to develop mammary and uterine cancer in life.
~A dog that isn't spayed before sexual maturity has a very good chance of developing the life-threatening uterine infection. Risk greatly increased after having puppies.

A reputable breeder:
Breeds ONLY for the improvement of the breed, and NO other reason.
Has pedigreed dogs with papers, and the puppies should be registered.
Has more than one breeding pair.
Doesn't breed more than two breeds (Infact two is pushing it if you ask me).
Makes sure their dogs are health certified (hips, elbows, eyes, etc.).
Breeds purebred dogs.
Discontinues any line that shows any sign of illness.
Keeps their dog competitive in shows such as conformation, agility, diving, flyball etc.
Breeds on demand and/or has the puppies reserved before he/she breeds.
Takes the time that is nesecarry to be sure that his/her dogs match up personality-wise before breeding.
Has their dogs checked atleast twice a year.
Makes sure that their dogs are socialized with other dogs and people.
Are responsible owners as well as breeders (takes dogs for walks, keeps them on a healthy diet etc).