I got the following from AAHA Pet Owner | AAHA
Answer: It's possible to spay a dog during estrus, or heat, but it's not the best time. Most veterinarians prefer not to spay a dog in estrus because the uterine tissue can be swollen, fragile, and more prone to damage. Dogs also have a tendency to bleed more when they're in heat. Because of this, estrus makes surgery more risky for dogs, as well as for cats.
Most veterinarians prefer to delay spaying a dog until she has been out of estrus for a month or so. And, since a recently spayed dog may still be attractive to males, performing the surgery during heat won't do anything to keep the neighbors' dogs out of your yard. The best thing for your dog's health is to keep her safe inside for now and to have your veterinarian perform surgery in a month or so.