Spaying While In Heat
During a heat cycle, the blood vessels that supply the ovaries and uterus are engorged. Additionally these tissues are more "friable" (may tear more easily than normal) and bleed more. Even tissue not associated with the reproductive organs - skin, fat, muscle - often seep more than normal during surgery.
This often makes a typically elective surgery more stressful than it needs to be, and carries additional risk of bleeding problems during the surgery or post-operatively. Some veterinarians will not spay a pet in heat and recommend that surgery be scheduled a week or two after the cycle ends.
Sometimes the answer is "maybe." Sometimes veterinarians are selective about the type of pet that they will spay while in heat. Generally speaking: Cat in-heat spays are easier than dog in-heat spays. Small dogs are easier than large dogs. Overweight pets are difficult to spay both in heat and not in heat. Anesthesia is riskier too. One more reason to make sure your pet is not overweight.
Additional Costs When Spaying a Pet in Heat
If a pet is spayed while in heat, the surgery takes a little longer and additional surgical supplies such as gauze sponges and suture may be needed. Fluids during surgery are standard for many practices and are built in to the surgery cost. For some practices, this may be an additional cost for an in-heat spay. (IV fluids during surgery always a good idea in my opinion.)
Dogs and cat are typically spayed at six months of age. If spayed before the first heat cycle (which happens around 6 months), the chances of mammary cancer are greatly reduced (96%). Spaying at any age eliminates the chance of ovarian and uterine cancers, as they are removed during a typical spay. Some animals, especially those at shelters, are spayed and neutered at much younger ages.
The "best" age to spay and neuter is a topic of much debate. Speak to your veterinarian about your specific pet (dog, cat, breed, etc.) for advice on age to spay. Most people will agree that it is best to avoid spaying when in heat if possible.
Sometimes an appointment is made in advance or an animal is found, rescued, etc. and the estrus status is not known when the surgery is scheduled. If this is the case, speak to your veterinarian about possible risks and added costs of spaying while in heat.