A female should be spayed before her first cycle ideally – though spaying at any age has more benefits than not spaying. While most vets prefer to perform the surgery in dogs not currently in heat, most surgeons will still spay a dog when in heat. There is often a small additional charge for this, as there is more time and attention needed during the surgery due to the increased blood supply to the uterus during a heat cycle. But, spaying a when in heat is far better than waiting and perhaps having other serious consequences, such as unwanted pregnancy, disease or illness contracted from other dogs she may be seeking to mate with.