An experienced veterinarian can spay a dog while she is in heat according to the ASPCA. The risk to the animal increases, and the operation is more complex and requires greater skill. These factors tend to make spaying while in heat more dangerous and more expensive than regular spaying.
During the heat cycle, the dog's ovaries and uterus are swollen, which causes more intense bleeding during surgery. If the dog loses a large amount of blood, she may die. In addition, the surgery is more expensive because it takes longer. If the veterinarian deems the procedure too risky, he or she may refuse to perform the operation.
For those choosing to wait until the dog's heat cycle passes before having her spayed, keep her indoors or leashed at all times to prevent unwanted pregnancies. The cycle usually lasts approximately 18 days.