Spay. Don't Litter

Today a woman brought in a small mixed breed dog. She thought the dog "might be pregnant", and for the last two days was "acting like she's trying to poop." The dog, an older unaltered female, was an "outside" dog, and the owner had no idea whether there had been a male visiting her or not.
X rays showed that there was one huge puppy in the birth canal. The dog was in critical shape, and we had to do an emergency c section. The puppy was huge, easily 2 or 3 times the size that a dog of this size could safely deliver, and had been dead for quite some time.
Although our price for these services is about a third of what the competition would charge, it was still a big bill that she really couldn't afford. She didn't want to put the dog down, but taking the dog home and doing nothing would have resulted in a slow, agonizing death for the dog, so she had to make a choice.
Had she had this dog spayed quite some time ago, she would have never been in this position.
Although she was ignorant about the condition her dog was in (this was her first female dog), I don't think she's a bad person or a bad owner. It is unrealistic to know what our pets are doing at every moment. With everything going on in our busy lives, we must leave them home alone for long periods of time, and it's impossible to know what all they've got up to. While some females have pretty obvious heat cycles, others have what's known as "silent heats" where the signs are only obvious to intact males.
So unless you have a pet that's a spectacular example of its breed (and no, having papers does not guarantee spectacular), stop the breeding potential before it starts. The idea of waiting for a first heat cycle or letting her have a litter of puppies first is a myth. This can be done when they're very young, about 6 months old. The local shelters will do the surgical alterations as young as two months, which is younger than most vets prefer, but taking care of it during the dog or cat's first year is best.