Is it OK to spay an adult cat that is in heat?
From the Editors of The Daily Cat
Ideally, a female cat should be spayed when it is 4 or 5 months old. In addition to stopping unwanted pregnancies, the procedure reduces the risk of uterine cancer, ovarian cancer and howling.
If you have a choice, it is not a good idea to spay a cat when it is in heat. At this time, there is swelling and increased blood flow to the cat's uterus. The operation may take additional time to perform and can be more difficult due to the increased blood supply. The hormonal peak at the time of heat may also complicate matters.
Your cat should be indoor-only, which eliminates many dangers posed by outdoor living, such as cars, people and diseases spread by other animals. If your cat is in heat, keep her inside and wait until the cycle ends before spaying her. If this is not possible for some reason, talk to your veterinarian about the problem. Often, animal hospitals will go ahead with the surgery to prevent the birth of unwanted kittens that might later become pregnant. It's a situation that can easily spiral out of control.